Final answer:
A mutation that prevents a signal molecule from binding to a transcription factor would result in constitutive expression of the structural gene in a negative repressible operon because the repressor would be unable to bind and inhibit transcription.
Step-by-step explanation:
In prokaryotic gene regulation, mutations affecting the binding of signal molecules to transcription factors can lead to different regulatory consequences depending on the type of operon involved. A mutation that prevents a signal molecule from binding a transcription factor in a negative repressible operon would result in constitutive expression of the structural gene. This is because, normally, the product of the biochemical pathway serves as a co-repressor, binding to the transcription factor and enabling it to repress the operon.
Without the ability to bind the signal molecule, the transcription factor cannot repress, leading to continuous, unregulated expression of the operon. Conversely, for a negative inducible operon, a similar mutation would result in non-expression as the operon requires an inducer to remove repression. In positive inducible or positive repressible operons, the binding of an activator or the prevention of such, respectively, would be affected, but these are not scenarios where constitutive expression would be expected as a result of the described mutation. Therefore, the correct answer is b. negative repressible.