Question

Fill in the blank with an appropriate modal (that expresses certainty):
You've been travelling all day. You ______ be very tired.

• A. could
• B. can
  
• C. must
• D. may
  

Answer 1

Answer: (C)

'Can' indicates ability / capacity.

'Could' indicates past ability / permission.

'May' indicates permission / possibility.

'Must' indicates certainty or surety.

Here, the condition of the listener is implied by the previous statement (that he/she has been travelling all day.) It may, hence, be assumed as certain that they would be tired. Therefore, the correct option is D: must.