P [6] =  2, 4

Q [11] = 2, 8, 1

R [15] = 2, 8, 5

S [17] = 2, 8, 7

Given the above is the electronic arrangement of PQRS elements, Q will form an electrovalent bond with S

Further explanation was provided by our student by name (Odebiyi Dare D-cent) via our facebook page. Thus he says:

without argument, D is the answer cos electrovalent involves the transfer of electron. Already, Q is 11 with elect configuration of 2,8,1, i.e having 1 valency electron which is ready to be loss by that atom of element Q, also, S is 17, with electronic configuration, with 2,8,7. There the 7valence electron of S attracts that nly 1 electron to attain a stable octet structure.