A strip is a type of security that allows an investor to separately purchase the principal portion and interest portion of the mortgage payments on a mortgage pool. In the context of the question, Robin Frazer has purchased a principal-only strip (PO) from GrossHaus Investment Bank. These PO strips are typically sold at a discount and increase in size over time as the principal component of the mortgage grows. This is in contrast to interest-only (IO) strips, which decrease in size over time as the principal due on the mortgage decreases. Therefore, the security that Frazer has purchased is a strip, making choice C the correct answer.

• Choice A is incorrect. A planned amortization class tranche (PAC) is a type of mortgage-backed security that is designed to protect investors from prepayment risk. It does not entitle the investor to the principal portion of a mortgage payment on a mortgage pool, but rather provides scheduled payments over a period of time.
• Choice B is incorrect. A collateralized mortgage obligation (CMO) is a type of security that pools together mortgages and then issues tranches to investors based on the risk profile and cash flow characteristics.
• Choice D is incorrect. A mortgage-backed security (MBS) is a broader category of investment that represents a claim on the cash flows from a pool of mortgage loans, but it does not specifically entitle the holder to only the principal portion of mortgage payments.