14 possible answers to
Answers with 3 characters: SET Answers with 4 characters: FIRM HARD PRIM TAUT Answers with 5 characters: STERN STIFF STONY TENSE Answers with 6 characters: STRICT Answers with 7 characters: ADAMANT Answers with 8 characters: CASTIRON IRONCLAD Answers with 9 characters: INELASTIC