Agreeable is different from the other three words. Agreeable (Adjective) means 'pleasant', 'giving pleasure', 'ready to agree'.

Firm (Adjective) means 'not yielding when pressed', 'fairly hard', 'definite', 'not likely to change' etc. 

Grave (Adjective) means 'serious and important', 'giving cause for worry'. 

Fastidious (Adjective) means 'selecting carefully', 'hard to please', 'easily disgusted'.

=> Ans - (C)