Answer
Since the lux genes are a group of genes in a bacterium which are coordinately regulated, they are probably organized into an operon which is turned on by some signal which builds up with bacterial population until some critical level is reached.
Work Step by Step
Review the lac repressor. See that this is a case (the hint is the one about the need for a quorum of bacteria) where something is turned on, so the analogy is the lac operon rather than the trp operon.
