Given the empirical formula, PCl​ 3​ , which of the following molecular formulas would be accurate?

A) P​ 3​ Cl​ 9
B) P​ 6​ Cl​ 12
C) P​ 9​ Cl​ 18
D) P​ 3​ Cl​ 18

The only one with a 1P to 3Cl ratio.

A is the only one with that ratio.