NaCl is the correct formula because sodium ions (Na+) and chloride ions (Cl-) combine in a 1:1 ratio to achieve charge neutrality. The other formulas propose incorrect ratios of the ions.

Magnesium nitride is correctly written as Mg3N2 because magnesium forms a +2 ion and nitride a -3 ion. Three Mg2+ ions combine with two N3- ions to balance the overall charge.

Al2O3 is correct because two Al3+ ions (total +6) balance with three O2- ions (total -6) to yield a neutral compound. The other options do not reflect the proper ion count needed for neutrality.

Potassium bromide is formulated as KBr because potassium (K+) and bromide (Br-) combine in a 1:1 ratio to achieve electrical neutrality. The other formulas imply incorrect ratios of the ions.

Calcium sulfide is written as CaS because the Ca2+ ion and the S2- ion balance each other in a 1:1 ratio. The other options do not provide the correct balance of charges.

Fe2O3 is formed by combining two Fe3+ ions (total positive charge +6) with three O2- ions (total negative charge -6) to yield a neutral compound. The other formulas do not reflect the correct stoichiometry needed for charge balance.

Copper(I) sulfide is correctly written as Cu2S because two copper ions (each with a +1 charge) are needed to balance the -2 charge of the sulfide ion. The other options do not maintain the necessary charge neutrality.

Aluminum sulfide is written as Al2S3 because two Al3+ ions (total +6) combine with three S2- ions (total -6) to achieve neutrality. The other formulas suggest an incorrect balance of ions.

Calcium (Ca2+) and phosphate (PO4^3-) combine in a ratio that requires three calcium ions and two phosphate ions to balance the charges, resulting in Ca3(PO4)2. Parentheses are used to group the polyatomic phosphate ion correctly.

Ammonium chloride is correctly written as NH4Cl because the ammonium ion (NH4+) and the chloride ion (Cl-) combine in a 1:1 ratio. The other options do not accurately represent the composition of the compound.

Zinc oxide is written as ZnO because zinc (Zn2+) and oxide (O2-) combine in a 1:1 ratio to achieve neutrality. The other options suggest an incorrect stoichiometric relationship between the ions.

Magnesium phosphate is formed by balancing three Mg2+ ions (total +6) with two phosphate ions (total -6), resulting in Mg3(PO4)2. The use of parentheses indicates that phosphate is a polyatomic ion and the ratios must reflect overall charge balance.

Iron(II) sulfate is written as FeSO4 because the Fe2+ ion and the sulfate ion (SO4^2-) combine in a 1:1 ratio. The additional coefficients or groupings in the other options are unnecessary for achieving charge balance.

Calcium carbonate is correctly written as CaCO3 because calcium (Ca2+) and carbonate (CO3^2-) combine in a 1:1 ratio. The other formulas either include extra subscripts or incorrect groupings that do not balance the charges.

Ammonium sulfate is written as (NH4)2SO4 because two ammonium ions (each +1) are needed to balance the -2 charge of the sulfate ion. Parentheses are used to enclose the ammonium ion, ensuring proper grouping.

Tin(II) sulfate is formed from Sn2+ and the sulfate ion (SO4^2-), combining in a 1:1 ratio to yield SnSO4. The other options include extra subscripts or incorrect multipliers that do not correctly balance the charges.

Barium (Ba2+) and dichromate (Cr2O7^2-) ions combine in a 1:1 ratio because their charges are equal in magnitude but opposite in sign. The correct formula is BaCr2O7, which reflects the exact ion ratio, unlike the other options.

Lithium phosphate is properly written as Li3PO4 because three Li+ ions (3 x +1 = +3) are required to balance the -3 charge on the phosphate ion (PO4^3-). This ensures the resulting compound is neutral.

Potassium permanganate is composed of one potassium ion (K+) and one permanganate ion (MnO4-), resulting in the simple 1:1 formula KMnO4. The other formulas suggest an incorrect stoichiometry.

Cobalt(III) ions (Co3+) and phosphate ions (PO4^3-) combine in a 1:1 ratio due to their equal and opposite charges, forming CoPO4. The alternatives incorrectly suggest different stoichiometric ratios that would not result in a neutral compound.