The series field of a short-shunt D.C. generator is excited by

The correct answer is A. external current.

The series field of a short-shunt D.C. generator is excited by an external current. This current flows through the series field winding, which is connected in series with the armature winding. The current through the series field winding creates a magnetic field that interacts with the magnetic field created by the armature winding. This interaction produces the output voltage of the generator.

The other options are incorrect because:

• B. armature current is the current that flows through the armature winding. It is not the current that excites the series field.
• C. shunt current is the current that flows through the shunt field winding. It is not the current that excites the series field.
• D. load current is the current that flows through the load. It is not the current that excites the series field.