In a

The correct answer is: D. solidly

In a A.C. series motor armature coils are usually connected to commutator solidly. This is because the commutator is used to reverse the current in the armature coils as the motor rotates, which creates a rotating magnetic field that drives the motor. If the armature coils were not connected to the commutator solidly, the current would not be reversed and the motor would not rotate.

Option A is incorrect because resistance would cause the motor to lose power. Option B is incorrect because reactances would cause the motor to vibrate. Option C is incorrect because capacitors would cause the motor to overheat.