Answer:
c. if sold short and not delivered within 13 business days of the trade, buy-in is required.
Step-by-step explanation:
According to the NYSE Regulation, a threshold Security is expressed by Rule 203(c)(6) of the SEC's Regulation SHO and Section 15(d) of the Exchange Act. In it is where it is stated that when a customer short sales that failed to deliver must be bought-in after 10 business days from the settlement, given that the security was on the exchange's threshold list as of the trade date under Regulation SHO and remains on that list for 10 business days past settlement, plus 3 additional days required from trade date to settlement date.
Hence, in this situation, I'm the correct answer is that Under Regulation SHO, a "threshold" security is one that ". if sold short and not delivered within 13 business days of the trade, buy-in is required."