Final answer:
In biological systems, DHFR is regulated by feedback control, specifically negative feedback, where the end product of a pathway can inhibit earlier steps to prevent waste and maintain balance.
Step-by-step explanation:
Regarding your question about the presence of DHFR (dihydrofolate reductase), it seems there has been a mix-up with the options provided. However, if we consider the context of control mechanisms within biological systems, DHFR would be present under a feedback control system. This particular control mechanism involves the regulation of a pathway by its end product. As indicated in your reference material, if a large amount of product D is formed, it can bind to Enzyme 1 to inhibit the further formation of intermediary products, thus preventing the waste of substrate A. This is an example of negative feedback, which is a common regulatory strategy in biochemical pathways to maintain homeostasis.