Though smaller than our solar system, a quasar, which looks like an ordinary star, emits more light ________ galaxy.

A. than an entire
B. than entire
C. than is entire
D. than entirely

The noun ‘galaxy’ is singular, so we need an article. In this case, we need the article ‘an.’
Option B and C are incorrect because they lack an article.
Option D is incorrect because we can’t use an adverb to modify a noun. Besides, it lacks an article.