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

Hint: We use a subordinating conjunction to join an independent and dependent clause.

Complete step-by-step answer:

 As given in the hint, subordinating conjunction is used to produce a single sentence by joining an independent and dependent clause.
An independent clause has a subject and a verb and it expresses a complete thought whereas a dependent clause expresses an incomplete thought.
So, in the above-given question: ‘You must come and see us’ is the independent clause, as it expresses a complete thought, whereas ‘You are in London again’ is a dependent clause, as it expresses an incomplete thought.
Now, we need to find suitable subordinating conjunction to join the two.
Let’s examine the given options:
a)When- It is a subordinating conjunction that is used to indicate the time during which action happens. Hence, it is the correct option.
b)Where- It is an adverb that is used to indicate the position of something. Hence, it is an incorrect option.
c)If- It is also subordinating conjunction but it is used to indicate something that happens on a condition. Hence, it is an incorrect option.
d)As- It is used to show a comparison between two things. Hence, it is an incorrect option.
The correct answer is Option ‘a’.

Note: You need to choose suitable subordinating conjunction depending on its function in a sentence.