Select the correctly punctuated sentence. it's time for the class to end but mr. johnson doesn't realize that. it's time for the class to end, but mr. johnson doesn't realize that. it's time for the class to end; but mr. johnson doesn't realize that.

The correctly punctuated sentence is:
it's time for the class to end, but mr.  johnson doesn't realize that.
the sentence is composed of two independent clause.
1) it's time for the class to end.
2) mr. johnson doesn't realize that.
these clauses are connected with the use of conjunction "but".
"but" is a coordinating conjunction, it goes between the items joined accompanied with a "comma".