Contraction risk in the context of mortgage-backed securities refers to the risk associated with the decrease in the expected life of a mortgage pool. This decrease is typically triggered by higher prepayment rates, which are often a result of falling interest rates. When interest rates fall, borrowers are more likely to refinance their mortgages at these lower rates, leading to a higher rate of prepayments. This, in turn, reduces the expected life of the mortgage pool, hence the term 'contraction risk'. This risk is a significant concern for investors in mortgage-backed securities as it can impact the timing and amount of their expected cash flows.

Choice A is incorrect. This choice incorrectly states that contraction risk is related to the increase in the expected life of a mortgage pool due to falling interest rates and higher prepayment rates. In reality, falling interest rates and higher prepayment rates lead to a decrease in the expected life of a mortgage pool, which is known as contraction risk.

Choice B is incorrect. This choice inaccurately describes contraction risk as being associated with an increase in the expected life of a mortgage pool due to increasing interest rates and lower prepayment rates. However, these conditions actually result in extension risk, not contraction risk.

Choice D is incorrect. This option wrongly suggests that contraction risk involves a decrease in the expected life of a mortgage pool due to increasing interest rates and lower prepayment rates. These circumstances would actually cause extension risk rather than contraction risk.