A Calculator Application is a benign program designed for arithmetic purposes and is not harmful. In contrast, Trojan Horse, Virus, and Worm are all forms of malicious software.

Antivirus Programs are designed to protect systems by detecting and removing malicious software, making them legitimate. The other options are forms of malware or potentially harmful software.

Performing standard system updates is a normal function of an operating system and not a behavior of malware. Malware typically exhibits self-replication, unwarranted advertisements, or unauthorized encryption.

Malware frequently exploits security vulnerabilities in the operating system to gain unauthorized access, while benign components like a calculator application are not targets. Exploiting OS vulnerabilities is a common tactic used by malicious software.

Benign programs operate transparently and perform expected functions without unauthorized interference. In contrast, malware performs actions such as self-replication, encryption, or data theft, which are outside normal program behavior.

Ransomware is known for encrypting victims' files and demanding payment for decryption. The other behaviors listed are characteristic of other types of malware.

A valid digital signature from a trusted publisher is usually a sign that the software is legitimate. Malware typically lacks proper digital signatures, uses obfuscation, or exhibits unusual file permissions.

Polymorphic techniques allow malware to change its code structure, making it difficult for signature-based antivirus programs to detect it. The other options are not common methods of evasion.

Regular software updates patch vulnerabilities that malware can exploit, thereby preventing many common infections. The other practices increase exposure to malware risks.

Worms are capable of self-replication without needing a host file, often spreading over networks independently. Viruses, on the other hand, require a host program in order to propagate.

Spyware is designed to surreptitiously collect sensitive information from users without their awareness. Its behavior is distinct from malware that deletes or encrypts files.

Adware typically bombards users with unwanted advertisements, which sets it apart from legitimate applications that offer maintenance or customization features. This behavior is a key indicator of adware.

Unexpected data transmission to unfamiliar servers can indicate that malware is communicating with a command-and-control center. Normal network activities, such as downloading from trusted sites, do not typically raise such concerns.

An unexpected file size combined with altered timestamps can be a red flag for tampering. Legitimate software typically maintains consistent file properties such as checksum and digital signature.

Rootkits are malicious tools created to conceal their existence while granting attackers persistent, privileged access to a system. This behavior distinguishes them from legitimate software utilities.

A Trojan that disguises itself as a useful tool is a common method for infecting systems, often resulting in degraded performance, unsolicited advertisements, and unauthorized system changes. Legitimate updates or productivity apps would not cause these symptoms.

Worms are designed to propagate across networks independently, often generating abnormal network traffic as they spread. The behavior described is most indicative of a worm infection rather than other types of malware.

Polymorphic code enables malware to continuously change its structure, helping it evade signature-based detection even when mimicking trusted elements. The other techniques serve different purposes and are not typically used for evasion.

Executable attachments, especially when unexpected in an email claiming to be an invoice, are common vectors for Trojan malware. The safest action is to avoid opening such files rather than risk executing malicious code.

A robust intrusion detection system monitors network activity for unusual patterns and can detect unknown threats, making it effective against zero-day attacks. In contrast, relying solely on signature-based detection or outdated systems leaves critical vulnerabilities unaddressed.