A compound adjective is formed when two or more words are joined together to form an adjective to modify the same noun. They are joined with a hyphen (-). we can form a compound adjective using numbers + a time period joined with a hyphen. E.g. The President of a company gave a 10-minute speech to the Board of Directors. Here 10-minute is a compound adjective where 10 is a number and minute signify a time period. They are joined with a hyphen. Thus, option A is the correct answer.