Explanation

First, you read that in the view of the commanding British officer, Adams and Hancock had committed “offenses,” so you might anticipate a word like offensive. Second, they were the only rebels whom he would not pardon or forgive, so he considered their behavior “unpardonable, unforgivable.” If you put these all together we’ll get something like “unforgivably offensive.”

Both flagitious and heinous match this meaning. Boorish means something like “crude,” but is much too weak a word to fit this sentence.

Hence options flagitious, heinous are correct!