Anyone know this question

Second option. 3

Explanation:

In order to balance a equation you need to have the same amount of atoms/moles in both sides.

Na₃PO₄  +  _ HCl  →  3NaCl  + H₃PO₄

In reactant side we have 3 Na and we have the same amount in product side.

We also have 1 P and 4 O in both sides of the equation.

As we have 3 moles of NaCl in product side, this means we have:

NaCl →  Na⁺  +  Cl⁻

3 moles of Na⁺ and 3 moles of Cl⁻. In conclussion we need 3 moles of Cl⁻ in reactant side.Then, the stoichiometry of HCl must be, 3.

3 moles of HCl means that we have 3 moles of protons (H⁺).

HCl  → H⁺  +  Cl⁻

If you see the phosphoric acid, we also have 3 moles of protons.

H₃PO₄ →  3H⁺  +  PO₄³⁻

Complete balanced equation is:

Na₃PO₄  +  3HCl  →  3NaCl  + H₃PO₄