Concept:An a.c. generator produces alternating current using slip rings, while a d.c. generator produces direct current using a commutator.
Explanation:In an a.c. generator, the rotating coil is connected to the external circuit through slip rings.
Slip rings maintain a constant connection, so the output current changes direction every half cycle.
To convert this into direct current, the slip rings must be replaced by a commutator.
The commutator reverses the external connection at each half cycle.
This reversal ensures that the current in the external circuit always flows in one direction.
Therefore, the required change is to replace slip rings with a commutator.
Answer:D. slip rings with a commutator