Answer

The correct choice:

400 meters.

Passing through the tunnel, the last car of the train moves a distance equal to the sum of the lengths of the train and the tunnel. In one minute the last car of the train moves through:

 60,000 / 60 = 1,000 meters.

Then the length of the train itself:

1,000 - 600 = 400 meters.

Answer: 400 meters.