Answer: a. U.S. Treasuries with 1 year to maturity
Step-by-step explanation:
The Government guaranteed the price of the carbon and the payoff is to be one year later.
The opportunity cost will therefore be a similar Government security to the payoff term of the carbon sale which is 1 year.
The Government security with a similar payoff term is the US Treasury bill with 1 year left till maturity and this will be the opportunity cost because instead of the Government issuing and paying out that security they will instead pay for the carbon.