C. Incomplete sentance. Suboridinate clause. The girl riding her bike is not enough to be a full sentance. Some things you could do to fix it: The girl was riding her bike. The girl riding her bike was very pretty. The girl was riding her new bike.
D. to relate a noun or pronoun to another part of the sentence. Example: A girl from home. The preposition 'from' shows the relationship between the girl and home.
