A combination of interest rate calls is referred to as an interest rate:
A is correct because an interest rate call is an option in which the holder has the right to make a known interest payment and receive an unknown interest payment. The underlying is the unknown interest rate. If the unknown underlying rate turns out to be higher than the exercise rate at expiration, the option is exercised. Thus, the interest rate paid will not be higher than the exercise rate. A combination of interest rate calls is referred to as an interest rate cap or sometimes just a cap.