The correct answer is D.

Subprime adjustable-rate mortgages (ARMs) are considered to be the riskiest type of mortgage-backed securities (MBSs). This is primarily due to the nature of the borrowers and the structure of the loans. Subprime mortgages are typically extended to borrowers who have poor creditworthiness, which is often reflected in higher loan-to-value (LTV) and debt-to-income (DTI) ratios. These borrowers are more likely to default on their loans, making the securities backed by these mortgages riskier. Additionally, the adjustable-rate structure of these mortgages adds another layer of risk. Unlike fixed-rate mortgages, ARMs have interest rates that can fluctuate over time. If interest rates rise, the borrowers' mortgage payments can increase significantly, further increasing the likelihood of default. Therefore, among the four options, subprime ARMs are the riskiest MBSs.

Choice A is incorrect. Prime fixed-rate MBSs are generally considered to be less risky than their subprime counterparts. This is because prime borrowers have a lower risk of defaulting on their loans, which reduces the overall risk of the MBS. Additionally, the fixed rate nature of these securities means that they are not subject to interest rate fluctuations, further reducing their risk profile.

Choice B is incorrect. Subprime fixed-rate MBSs carry more risk than prime fixed-rate MBSs due to the higher likelihood of default by subprime borrowers. However, they still have a lower level of risk compared to adjustable-rate mortgages (ARMs), as ARMs expose investors to additional interest rate risks.

Choice C is incorrect. While prime adjustable-rate mortgages do carry some additional interest rate risks compared with fixed-rate mortgages, they are still less risky than subprime ARMs due to the lower default rates associated with prime borrowers.