Correct option is B
In a DC series motor, the speed is inversely proportional to the flux, which is directly proportional to the current through the field winding.
N∝1/ϕ and ϕ∝I
· When starting a DC series motor without a load, the armature current is very small because there is no opposing load torque.
· Since flux (ϕ) is proportional to the current (I), the flux becomes very weak at low current. This causes the speed to rise excessively, leading to a phenomenon called runaway speed.
