The correct option is
A Uncommon
A synonym is a word or a phrase that is similar in meaning to another word/phrase.
Option B: 'Rare' means 'not occurring very often'.
'Uncommon' means 'not happening very often'.
Hence, 'uncommon' means the same as 'rare'. So option B is correct.
Option A: 'Common' means 'occurring or done often'. It means the opposite of the word 'rare'.
Option C: 'Important' means 'of great significance or value'.
Option D: 'Different' means 'not the same as another'. It means 'not the same', whereas 'rare' means 'not like others'. There's a slight difference in meaning.
These words do not mean the same as 'rare'. Hence, options A, C and D are wrong.