What is the basic function of catabolic operons like the lac operon?

a. to find an alternative energy source when glucose is absent or not present in sufficient amounts

b. to find an alternative source of carbon when glucose is absent or not present in sufficient amounts

c. to find an alternative source of carbon regardless of the amount of glucose present

If your on plato the answer is B

I don't believe it is C, because the lac operon is only switched on when there no glucose available, or it is only in trace amounts. This answer is suggesting the lac operon could be switched on at any time, which is not correct.

I'm tossing up between A and B, but I believe B is more specific, because lactose is a source of carbon, acting as an energy source.

I would go B.