What would be the most likely indication that a shell-and-tube lube oil cooler had in internal leak permitting mixing of the two fluids?

Correct Answer: Option A — Dropping level and frequent need to add make-up oil to engine sump.

Lube oil coolers typically operate at higher pressures than the cooling medium. An internal leak permits oil to escape into the cooling water, resulting in a loss of sump level. Frequent make-up oil requirements are the primary operational indicator of a breach in the heat exchanger's integrity.