Final answer:
The situation described is an example of a novation, where one party in a contract is replaced with another, with all original parties agreeing to the change. So, the correct answer is option b.
Step-by-step explanation:
When Philip can no longer fulfill his contractual obligation to mow Jessica's lawn due to an injury and Paul agrees to take over Philip's duties with Jessica agreeing to this change, Jessica must substitute Philip's name with Paul's on the contract. This situation is an example of a novation, which is a legal act that replaces an existing obligation with a new one, effectively replacing a party in the original contract with a new party. So, the correct answer is option b.