The neural crest is those temporary sets of cells that are stick to particular vertebrates that arise due to the embryonic layer of germs. Neural crest provides a way to produce PNS(peripheral nervous system).
These cells are migratory cells that can develop into different cells tissue all over the body.
A dihybrid cross would be crossing 2 (usually linked) traits in the same punnett square, while a trihybrid cross is with 3 traits. Since he's only studying 1 trait (tall vs. short) the answer is C, a monohybrid cross.