Question

You will be in London again. You must come and see us then.
You must come and see us _____ you are in London again.

• A. when
  
• B. where
• C. if
  
• D. as
  

Answer 1

The correct option is A when

'When' is used to introduce a single completed event that takes place in the middle of a longer activity or event. Here the same is indicated. Therefore, the correct option is: when. We cannot use 'if' since it is certain that the person will be in London, evident from the sentence 'You will be in London again'.