I �d say none of the above and here are the reasons:
If you want answer a to be the correct one the sentence has to be:
There is exactly a ............... students in the college.
If you want answer b to be the correct one
You should erase the word: exactly and add of:
There are thousands of students in the college.
If you want answer c to be the correct one the sentence should be:
There is exactly .................. students in the college.
If you want answer d to be the correct one exactly and add of:
There are hundreds of students in the college