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

Answer: Option D

We do not use any article before the names of certain diseases such as measles, mumps, cholera, malaria etc.

These diseases are considered uncountable and general in meaning, so they take no article.

For example: “He is suffering from malaria.” / “She has measles.”

Therefore, the correct sentence is: “Peter has a fever because he has measles.”