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

• We use a subordinating conjunction to join an dependent and independent clause. Example:- because,even if,once are some examples of the subordinating conjunction.
• Firstly, we have to look at each of the options and check which option is a subordinating conjunction. If all of them are then we will check one by one by placing them in the blank which one makes sense.
• Here when and if are the subordinating conjunction.
• If is used in the condition sentence, so it is not a suitable option.
• When is also a subordinating conjunction that is used to indicate the time during which action happens.

Hence, option A is the correct answer.