Final answer:
Both Technician A's suggestion of a failed horn relay and Technician B's suggestion of a short to ground in the circuit could cause the horns to only work when a jumper wire is used, making (C) Both A and B the correct answer.
Step-by-step explanation:
When horns in a vehicle only operate if a jumper wire is connected between terminals 30 and 87 of the horn relay, it suggests there might be an issue with the relay system. Technician A suggests that a failed horn relay could be the cause, which is a possibility because if the relay is not functioning, the circuit will not complete, and the horns will not receive power unless bypassed with a jumper wire. Technician B mentions that a short to ground in the circuit between the horn relay and the horn button could also be the cause. A short to ground would likely prevent the relay from activating normally, necessitating the use of a jumper wire for the horns to function.
Therefore, the correct answer is (C) Both A and B, as either a failed relay or a short to ground could be responsible for the need to use a jumper wire to activate the horns.