The correct option is D possess
'Relinquish' means 'voluntarily cease to claim or own'. For example, 'to relinquish one's assets.'
Option D: 'Possess' means 'to own something'. It means the exact opposite of the given word. Hence D is correct.
Option A: 'Convert' means 'to change form or character'.
Option B: 'Condense' means 'to make something denser or more concentrated'.
Option C: 'Longest' is a superlative adjective that speaks of length.
These words do not express the opposite meaning of the given word. Hence A, B and C are incorrect.