Final answer:
The prohibited activity among those listed is overselling an IPO and repurchasing the shares for a profit, known as greenshoe option which is only allowed to prevent immediate price drop. Stabilizing an IPO and publishing favorable research are permitted under certain circumstances, while short-tendering stock is not allowed.
Step-by-step explanation:
The question pertains to the activities associated with initial public offerings (IPOs) and following rules and regulations in the financial market. Of the options provided, overselling an IPO and subsequently repurchasing the shares in the secondary market at a profit (option b) is not permitted. This is known as greenshoe option which is only allowed to stabilize the market price and prevent immediate drop after the IPO.
Stabilizing an IPO at the public offering price or below (option a) is a legal activity executed by underwriters to maintain the market price of a new issue. Publishing favorable research about a company (option c) shortly before a follow-on offering can influence the market, and while potentially unethical, is not always forbidden. Short-tendering stock (option d) is the prohibited activity, as it involves selling stock that one does not own during a tender offer which is against the rules.