When a glass rod is rubbed with a piece of silk cloth the rod

Option (a) when an object is charged by rubbing it against another object the two objects gets oppositely charged. Hence the same charge cannot be there on both rod and the cloth.

Option (b) by convention, it is considered that the charge acquired by the glass rod is positive and charge acquired by cloth is negative. Therefore, the rod becomes positively charged while the cloth has a negative charge. Hence (b) option is correct.

Option (c) when an object is charged by rubbing it against another object the two objects gets oppositely charged. Hence the same charge cannot be there on both rod and the cloth.

Option (d) by convention, it is considered that the charge acquired by the glass rod is positive and charge acquired by cloth is negative. Hence this option is not correct.