Sandra is . . . . . . . . better dancer than . . . . . . . . singer.

In expressing a comparison, if two nouns refer to the same person or thing, the article is used before the first noun only. But if the nouns refer to different persons or things, the article is used before each noun. Since in the given statement, both the nouns dancer and singer refer to one person the article will be used before the first noun only and no article will be used before 'singer'. Also, the indefinite article 'a' should be used whenever someone's work or job is being referred to. Therefore Option D is the correct answer.