they are used for AC

because in ordinary ammeters torque is proportional to current (i)

but in hot wire ammeter torque is directly proportional to i²

so that  direction change in current is nullified

this arrangement is used because in AC, current changes its directon in every half cycle. so net magnitude of torque becomes zero.

so hot wire ammeter is designed

yes i do agree with u but the correct  ans is option c.........