Answer: The answer is C, a monohybrid cross.
Explanation:
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.
DNA fingerprints help the police by providing the information needed to compare the DNA found at the crime scene to any documented DNA of any person through their data base to find the criminal.