The correct answer is Option B.
As a rule, a singular subject/noun is followed by the plural form of the relevant verb.
HE is a single person watching television. Hence, WATCHES is the correct word that completes the sentence.
The remaining options are incorrect because:
Option A: WATCH can be used if the subject plural in number.
Option C and D: SEE and BUY are incorrect verbs associated with the word TELEVISION.
Hence Option B is the correct answer.