A swaption is an option that gives the holder the right to enter into a swap agreement with a predetermined fixed rate in exchange for a floating rate at a future time. This instrument is most suitable for Coelho's situation as it allows him to enter into the swap agreement on specified terms at a predetermined date. Swaptions are essentially options on forward swap agreements. They provide the holder with the right, but not the obligation, to enter into a swap agreement at a future date. This gives the holder flexibility and protection against adverse movements in interest rates. Swaptions can be used for a variety of purposes, including hedging interest rate risk, speculating on future interest rate movements, and enhancing portfolio yield. They are commonly used by corporations, financial institutions, and investment funds.

Choice A is incorrect. An extendable swap allows the holder to lengthen the duration of an existing swap at a predetermined date, which does not align with Coelho's need to enter into a new swap agreement at a future date.

Choice B is incorrect. A callable swap gives one party the right but not the obligation to terminate the contract before its maturity, which again does not meet Coelho's requirement of entering into a new agreement in future.

Choice D is incorrect. A constant maturity swap involves swapping fixed interest payments for floating rate payments linked to an interest rate index with a continuously changing (or "constant") maturity.