The correct option is
B Rarely
A synonym is a word/phrase that means the same as another word/phrase.
Option B: 'Seldom' means 'on only a few occasions, not often'.'Rarely' means 'not often'.
As you can see, 'rarely' means the same as 'seldom'. Both mean 'on very few occasions'.
Hence, option B is correct.
Option A: 'Always' is 'on every time, every occasion'. It means the opposite of the given word 'seldom'. It's an antonym and not the synonym.
Option C: 'Later' means 'afterward, at a time in the future'.
Option D: 'Often' means 'many times, frequently'.
This too means the opposite of 'seldom'.
These words do not mean the same as 'seldom'.
Hence, options A, C and D are wrong.