Correct Answer - Option 2 : armature current only
For DC Machine, torque is directly proportional to armature current and flux.
T ∝ ϕ Ia
In dc shunt motors, flux is constant (not dependent on armature current). Hence torque is proportional to armature current.
T ∝ Ia
In dc series motors, flux is directly proportional to armature current. Hence torque is directly proportional to square of the armature current.
Ta ∝ Ia2
In dc series motor at saturation condition, flux is constant. Hence torque is proportional to armature current.
T ∝ Ia
Where, T = torque, Ia = armature current, ϕ = field flux
