Why the answer is B, and why the others tempt you.
'Cough' is pronounced /kɒf/, so it rhymes with 'off' /ɒf/. Despite the shared spelling, 'through' ends in /uː/, 'bough' ends in the diphthong /aʊ/ and 'though' ends in /əʊ/. The '-ough' spelling is a classic test of sound versus spelling.
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