The correct answer is D. no article.
The article "a" or "an" is used before a singular noun that refers to a general thing or person. In this case, Sheila is watching television, which is a general thing. Therefore, we do not need to use an article.
The article "the" is used before a singular noun that refers to a specific thing or person. In this case, Sheila is not watching a specific television show or channel. Therefore, we do not need to use the article "the."
Without an article, the sentence "Sheila spends most of her free time watching television" is more general. It could mean that Sheila watches any type of television show or channel, or that she watches television for a long time.
