If a train conductor is describing the speed of the train as well as the direction the train is going, then the train conductor is said to be describing the train's. a. acceleration. b. speed. c. velocity d. magnitude

Since the conductor is describing is speed as well its direction, he's describing train's velocity.

Velocity is a vector quantity i. e it depends on both direction as well magnitude.

Speed is the magnitude of velocity.

Hence C is correct.