Four elements P, Q, R, and S have atomic numbers 12, 13, 14, and 15, respectively. What is the valency of Q?

Valency is the tendency of an atom to form bonds with other atoms using its valence or outermost shell electrons. Electronic configuration of Q = 2, 8, 3.

Since, three electrons are present in its outermost shell, the valency of Q is 3 because it can lose 3 electrons to attain a noble gas configuration. 
The valency of Q is 3⁺.