The right answer is the tryptophan itself.
The tryptophan operon has a regulatory gene, trp R (located upstream of the trp operon) is a constitutive gene that is transcribed and permanently translated into an inactive monomeric protein, the apo-repressor. The latter has 2 binding sites, a tryptophan binder and the other the operator.
Synthesis of the operon mRNA is controlled by the apo-repressor, which blocks transcription when bound by tryptophan (which acts as a co-repressor).
