What are the correct coefficients to balance the given chemical equation? ...H3PO4(aq) (aq) + KOH(aq) (g) rightwards arrow H2O(l) (s) + K3PO4(aq)

Iknow the answer is a but i don't know why

The big bang nucleosynthesis theory states that elements were produced in the first few minutes of the big bang while elements have their origins in the interiors of stars, forming much later in the history of the universe.

What is a property of a double replacement reaction