Explanation

The sentence uses 'therefore' to indicate a logical consequence. If a person is diligent (hardworking), the natural result is success, not failure. Options (a) 'he will not succeed' and (b) 'he can't succeed' are negative and contradict 'therefore'. Option (d) 'he succeed' is grammatically incorrect (needs 'will' for future). Option (c) 'he will succeed' is the only logical and grammatical completion. The blank cannot be negative since the person is hardworking. Hence option C is correct. Cross-checked with Oswaal CDS Yearwise Solved Papers, whose explanation confirms the positive consequence.