Do you know what the matter is?

[Yuliya Guseva]

Hello!

"Do you know what the matter is with your visa application?"

Is this sentence correct?
Is it formal or informal?

[emsr2d2]

I would use "problem", not "matter". It's neither formal nor informal. It's OK and understandable. You will hear "Do you know what the problem with your visa application is?" (note the position of the word "is").

[BobK]

Or, much less formally, 'Do you know what's up with your visa application?'

b