Question

Fill in the blank with a suitable Modal Auxiliary:
That man on the motorbike isn't wearing a helmet. That's dangerous.
He ________ wear a helmet.

• A. will
• B. can
• C. must
• D. should

Answer 1

Answer: (D)

should

The given sentence expresses an obligation. He is obliged to wear a helmet because it's dangerous if he doesn't.
Option A: 'Will' refers to a future action. The given sentence does not describe an action that is yet to happen. Hence it does not complete the sentence in meaning.
Option B: 'Can' refers to possibility or permission. The sentence refers to neither over here.
Hence options A and B are incorrect.
Options C and D: 'Must' and 'should' have similar meanings. Both refer to an obligation or something that one is supposed to do.
But, 'must' is a stronger word when compared to 'should'. 'Must' is used in terms of law, or a set rule of nature. 'One must pay for his deeds'.
'Should', in comparison, says what someone is obliged to do, what someone is morally supposed to do.
'Must' means that someone 'has to do it', come what may.
For this sentence, 'should' is more preferable than 'must', as the former refers to obligation; the latter expresses compulsion.
Hence option D is correct and C is incorrect.