Final answer:
A man posting a $100 reward for anyone who returns his missing dog is an example of a unilateral contract.
Step-by-step explanation:
The correct answer is A. a unilateral contract.
A unilateral contract is a type of contract where one party is bound to perform a specific action upon the occurrence of a certain event. In this case, the man has posted a $100 reward for anyone who returns his missing dog. By posting the reward, he is making an offer to enter into a contract with anyone who finds his dog and returns it. The contract is formed when someone finds the dog and performs the requested action of returning it.