Discussion

Discussion
 

-Then we will have:cool: group discussion (no the here) :cool:about Questions four and five. (=the act of discussing things)
-Then we will have:cool: a group discussion :cool:about Questions four and five. (=a period of discussing things)
-Then we will have:cool: group discussions :cool:about Questions four and five. (=just the plural of the above)

Do you agree with me?

You are right about the second and third choices.
The first choice is not quite right although it would be understood. The first choice may be more acceptable in British English, but that would need to be verified.

Do you mean 'group discussion' without a/the is wrong?

That's correct. You must use an article.