Liquid mercury reacts with nitric acid to produce aqueous mercury(II) nitrate, nitrogen monoxide gas and water. The solution is acidic. Neither water nor nitrogen monoxide will ionize.

Strontium has a charge of 2+ as an ion, and a charge of -3. So to get an equal charge, the least common multiple is 6, so you would need 3 Strontium ions and you would need 2 phosphate ions. Keep in mind that you have to multiply the entire ion, so you would need to have , NOT  because then you would only have 1 phosphorous, and that would not be phosphate anymore.
So the correct one is #4