This word means the same as the given italicized word. Hence option B is correct.
Option A: 'Harmful' means 'causing or likely to cause harm'.
Option C: 'Unrewarding' means 'not rewarding or satisfying'.
Option D: 'Unpopular' means 'not liked or popular'.
These words do not mean the same as the given word 'obsolescent'.
So, options A, C and D are wrong.