He usually listens to the radio but at the present moment he . . . . . . . . television.

Answer: Option C

In the sentence, the words 'at present' clearly indicate that the tense of the sentence is present tense. Also, the sentence indicates that the subject's activity is currently ongoing. Thus, the answer must be in the present continuous tense.
Option C is correct. 'is watching' is in the present continuous tense. It indicates that the subject usually listens to Radio, but he/she is currently watching TV. Thus, it is the correct answer.