Which of the following is NOT a feature of a tracker mortgage?

The answer is correct because a tracker mortgage is specifically designed to have its interest rate fluctuate in line with a related benchmark, typically the central bank's interest rate, such as the Bank Rate. This means that as market rates change, so too does the interest rate on a tracker mortgage, resulting in varying monthly payments for the borrower.

In the context of the other features, the interest rate directly following market rates (a) and the rate rising or falling based on the Bank Rate (b) are fundamental characteristics that define how tracker mortgages operate. Additionally, the feature indicating that the mortgage's cost can change automatically (d) aligns with the inherent nature of these types of mortgages, as they automatically adjust to reflect changes in the underlying rate.

Given that the correct answer identifies a feature that is not aligned with the nature of tracker mortgages, it effectively highlights that borrowers do not pay a fixed amount regardless of market changes; instead, their payments will vary as the interest rate changes in response to market conditions.