Explanation

The underlined word ‘redoubtable’ means formidable — commanding respect through a fearsome reputation — which is the wrong word here. The sentence needs a word meaning ‘open to doubt’: ‘doubtful’ (of doubtful integrity = of questionable honesty). ‘Doubting’ describes one who doubts, and ‘doubtless’ means certain — the opposite of what is needed. Hence (b) is correct.