Under Regulation M, which of the following activities is NOT permitted during the restricted period for a distribution?

Why C — Purchasing shares in the open market by a participating underwriter for its proprietary trading account unrelated to stabilization

Regulation M Rule 101 prohibits underwriters and other distribution participants from bidding for or purchasing the offered security during the restricted period, except for specifically permitted activities such as stabilizing bids, syndicate covering transactions, and penalty bids. Proprietary trading purchases unrelated to stabilization or covering would violate this prohibition. The restricted period generally begins one or five business days before pricing (depending on the security's trading volume and public float) and ends when the distribution is complete.