The correct answer is D. no article.
The sentence “I thought there was somebody in the house because there was light on inside” is about a general situation, not a specific one. Therefore, we do not need to use an article.
If we were to use an article, it would change the meaning of the sentence. For example, if we used “a” light, it would mean that there was one light on inside the house. If we used “an” light, it would mean that there was a single light on inside the house, but we do not know what kind of light it was. If we used “the” light, it would mean that there was a specific light that we were talking about.
In this case, we do not need to specify which light we are talking about, so we do not need to use an article.