(a) P and R
(b) Q and S
(c) P and S
(d) R and S
Solution:
Option (c) is the answer because the atomic numbers of the four elements P, Q, R, and S are 6,10, 12, and 17 respectively. P (6= 2,4) and S (17= 2,8,7) are the two elements that can interact to produce covalent compounds.