Final answer:
Tryptophan biosynthesis is regulated by repression through the tryptophan operon; when tryptophan levels are high, it binds to a repressor, inhibiting further synthesis.
Step-by-step explanation:
The pathway that is regulated by repression caused by the accumulation of its own end-product is D. Tryptophan biosynthesis. The tryptophan operon is a repressible operon which is normally active but can be inhibited if the end-product, tryptophan, is present in sufficient quantities.
This is a regulatory mechanism where tryptophan itself serves as a corepressor. When its levels are high, it binds to the repressor protein which then binds to the operator site on the operon, thereby preventing transcription of the genes needed for further synthesis of tryptophan. This form of regulation prevents the cell from wasting resources on producing something it already has enough of.