posted by rfvv .
1. I am busy studying for the mid-term exam.
2. I am busy studying for the midterm exam.
3. I am busy studying for the mid-term exams.
4. I am busy studying for the mid-term exams.
(Here in our country, During the period of the mid-term exam(s), students have tests for all the subjects, about 10 subjects. So they have to prepare for all the tests. In this case which expressions are grammatical?)
3 and 4 are the same and are correct for the multiple-exams situation you described.