The correct answer is D. no article.
The word "expert" is a noun that describes someone who has a lot of knowledge or skill in a particular area. In this case, the speaker is describing their sister as an expert in bike racing. Since the speaker is not specifying which expert their sister is, they do not need to use an article.
The article "a" is used before singular nouns that refer to a general or unspecified person or thing. For example, "I saw a man walking down the street." The article "an" is used before singular nouns that begin with a vowel sound. For example, "I saw an elephant in the zoo." The article "the" is used before singular nouns that refer to a specific person or thing that has already been mentioned or is known to the listener. For example, "I saw the man who stole my car."
In this case, the speaker is not specifying which expert their sister is, so they do not need to use an article.
