Answer: The only possible suffix for the word "regret" is "-able".
Explanation: A suffix is a morpheme that is affixed at the end of a word. A suffix can add grammatical information or lexical information to a word. "-Able", "-ible", "-ation", "-ance" and "-ence" are all suffixes; however, only "-able" can be added at the end of the word "regret" in order to form the word "regrettable". The meaning of the suffix "-able" is having the power to do something; therefore, the word "regrettable" refers to something that has the power to cause regret.