Final answer:
The mutation that causes the repressor to no longer bind, leading to constitutive synthesis of lactose-utilizing proteins, occurs in the operator of the lac operon.
Step-by-step explanation:
A mutation that changes the nucleotide sequence of the operator, so that the repressor is no longer able to bind to the site, will cause constitutive synthesis of the lactose-utilizing proteins. The correct answer to the question is 'a) Operator.'
In absence of lactose, the repressor protein binds to the operator, preventing RNA polymerase from transcribing the operon. If lactose is present, it is converted to allolactose, which binds to the repressor and causes it to dissociate from the operator, thereby allowing transcription. A mutation in the operator sequence that prevents the repressor from binding would similarly lead to continuous transcription of the operon, known as constitutive synthesis, even in the absence of lactose.