This is a question from a text book. "Mr. Thompson wrote to the main office to find out ( ) his budget would be for the following year." There are 4 options given. (A) what (B) who (C) when (D) it The answer (A) is given in this case. I can agree with that, but I also feel that "(C) when" can be used in this case. If it is not allowed to "when" grammatically, could you please advise me the reason.
You can use "when" to introduce a single completed event that takes place in the middle of a longer activity or event. In these cases, you usually use a continuous verb in the main clause to describe the background event:
He was walking back to his flat when he heard an explosion.
Depending on the context, when can mean ‘after’ or ‘at the same time’.
When you open the file, check the second page. - when meaning ‘after’
I eat ice cream when I am on holiday. - when meaning ‘at the same time’
We often use "just" with "when" or "as" to express things happening at exactly the same time:
The phone always rings just when I’m closing the front door.
She was a brilliant gymnast, but she had a terrible accident in 1999, just as her career was taking off.