Identify the mortgage type that has an interest rate which changes in accordance with market interest rates.

A variable rate mortgage is characterized by an interest rate that fluctuates with market interest rates. This means that as the market rates rise or fall, the interest rate on the mortgage adjusts accordingly. Homeowners with this type of mortgage can benefit from lower initial rates when market rates are low, and they may face higher payments over time if interest rates increase. This mortgage type is appealing for borrowers who may want to take advantage of potential decreases in interest rates, but it also comes with the risk of rising payments if rates go up.

In contrast, a fixed-rate mortgage maintains a constant interest rate throughout the life of the loan, providing stability in payments but not allowing for adjustments based on market conditions. The convertible mortgage allows for potential adjustments to the mortgage type after a certain period but does not inherently adjust interest rates in accordance with the market. A shared appreciation mortgage involves sharing the appreciation of property value rather than directly linking interest rates to market fluctuations. Therefore, the unique aspect of a variable rate mortgage is its direct connection to market interest movements, making it the correct answer in this context.