In diesel engine, ignition is caused by

The correct answer is C. Compression.

In a diesel engine, ignition is caused by compression. The air-fuel mixture is compressed until it reaches a temperature high enough to ignite without a spark. This is in contrast to a gasoline engine, which uses a spark plug to ignite the air-fuel mixture.

Option A is incorrect because a spark plug is not used in a diesel engine.

Option B is incorrect because an automatic starter is used to start the engine, not to ignite the air-fuel mixture.

Option D is incorrect because friction does not cause ignition in a diesel engine.