Final answer:
The proper fix for a hole in the coolant recovery tank is to replace the tank. This ensures a reliable and correct function of the cooling system, avoiding temporary fixes such as stop leak or unreliable methods like plastic welding.
Step-by-step explanation:
If the coolant recovery tank has a hole in it, the proper fix would be option C, to replace the tank.
Adding stop leak (option A) is not advisable as it can clog other parts of the cooling system and is generally considered a temporary fix. Option B, doing nothing, is not a solution as the recovery tank is essential for capturing excess coolant and preventing leaks. Option D, plastic weld the hole closed, is also not a recommended fix because it may not seal properly, and the structural integrity of the tank could be compromised which could lead to further leakage or failure under pressure.
Therefore, the most professional and long-lasting repair would be to replace the coolant recovery tank which ensures that the cooling system will function correctly and reliably.