Which class

It depends entirely on which (by country) system you're dealing with. Terms vary from class to form to year to level. In the US we refer to elementary education as 1st grade, 2nd grade, etc. Then in high school, we have freshman, sophomore, junior and senior years/classes. The same applies to college and university, although 'class' is less frequent there.

Here's a Canadian answer.

Q - Which class are you in this year?
A - Grade 10.

CliveHere's a Canadian answer.Q - Which class are you in this year? A - Grade 10.

Here in your answer .... A - Grade 10. What does " A " refer to?
probably

PhilipIt depends entirely on which (by country) system you're dealing with. Terms vary from class to form to year to level. In the US we refer to elementary education as 1st grade, 2nd grade, etc. Then in high school, we have freshman, sophomore, junior and senior years/classes. The same applies to college and university, although 'class' is less frequent there.

sunny123 . A - Grade 10. What does " A " refer to?

Q = Question
A - Answer

Then the correct way to say is ....

A: Which class are you in this year?
B: Grade 10
Right?

Right.

But Clive, the question could be .....

A: What grade are you in this year?
B: Grade 10

or

C: Which class are you in this year?
D: Grade 10
Are both ways correct?
Are both ways asking the same thing?

In Canada, up to the age of 18 we speak of grades.
In post-secondary education, we speak of classes and years.

Clive

PhilipIt depends entirely on which (by country) system you're dealing with.

That's a good point. The US terminology of "grade" as well as "freshman, sophomore, junior and senior" are not used in the UK.

There is no single, correct answer that can be given. It's going to depend on which part of the world is the context.