Question

Fill in the blank with the most suitable option:
I'm coming to London tomorrow. I'll ring you _______ I arrive.

• A. while
• B. when
• C. until
• D. if

Answer 1

The correct option is B when

Answer : B

Subordinating Conjugation : It is a word or phrase used to link a dependent clause to an independent clause.

It is used to:

• show a cause-and-effect relationship between the dependent & subordinate (independent clause)
• to show a relationship involving a transition of time or place between two clause

Let's take a look at the usage of the subordinating conjugations in the options in relation to time:

• while : It is used to express an action in progress; sometimes this action might be interrupted
  
• when : It is used to refer to a particular time; at or during the time that; also it is used to show that one event happens slightly after another
• until : It is used to express the time when an event or action comes to an end
• if : It is not used to show a relationship of time but is used to introduce a conditional clause

Here in the question, we understand that the speaker is expressing the first person will call the second person upon his/her arrival to London. Thus, first he would arrive at London & then would ring the other person. Thus this shows two events occurring with a slight time difference.

Therefore from among the options, 'when' is the most suitable subordinating conjugation to complete the sentence.

Thus, the correct complete sentence is:

I'll ring you WHEN I arrive.