Question

some bacteria are metabolically active in hot springs because...
they are able to maintain a lower internal temperature.
high temperatures make catalysis unnecessary.
their enzymes have high optimal temperatures.
their enzymes are completely insensitive to temperature.

Explanation:

Enzymes are proteins that catalyze biochemical reactions. Each enzyme has an optimal temperature at which it functions best. Bacteria in hot springs have enzymes with high optimal temperatures, allowing them to be metabolically active in the high - temperature environment.
Maintaining a lower internal temperature is not the key factor as the external temperature is high and they need to function in that. High temperatures do not make catalysis unnecessary as enzymes (catalysts) are still required for most biochemical reactions. Enzymes being completely insensitive to temperature is incorrect as all enzymes have some temperature - sensitivity range, just that for these bacteria, the optimal is high.

Answer:

their enzymes have high optimal temperatures.