. . . . . . . . television was on but nobody was watching it.

The correct answer is D. no article.

The article "a" or "an" is used before singular countable nouns that are not specific. In this case, the noun "television" is singular and countable, but it is not specific. We do not know which television is on, so we do not use an article.

The article "the" is used before singular countable nouns that are specific. In this case, the noun "television" is singular and countable, but it is not specific. We do not know which television is on, so we do not use "the".

No article is used before plural nouns, uncountable nouns, and nouns that are not specific. In this case, the noun "television" is singular and countable, but it is not specific. We do not know which television is on, so we do not use an article.