Peter has a fever because he has . . . . . . . . measles.

The correct answer is D. no article.

The word "measles" is a common noun, which means it does not need an article. Common nouns are general terms that refer to any member of a group. In this case, the word "measles" refers to any person who has the measles virus.

The articles "a" and "an" are used before singular common nouns that refer to a specific person, place, thing, or idea. In this case, the word "measles" does not refer to a specific person, so it does not need an article.

The article "the" is used before singular common nouns that refer to a specific person, place, thing, or idea that has already been mentioned. In this case, the word "measles" does not refer to a specific person or place, so it does not need the article "the."