Answer: b. a counterclaim.
Explanation: a counterclaim is a claim made to rebuke allegation against you. If you are sued for breaching a contract and you, in turn, also file suit against the plaintiff and claim that he/she was really the one who breached the contract, your claim against the original plaintiff is known as a counterclaim.
In this case Mabel denies Lauren charges and claims Lauren breaches their original agreed contract.