Option A is totally false, as both queries output same results.
Option B seems right, but projection never allows duplicates(in relational algebra). So, option B is obviously false(The only confusion is if we treat the question as part of SQL, then projection is same as select and it produces duplicates.
Option C also seems right.
If we have x=5000, all tuples are already OK. We don't need to filter anything, but still, SQL don't know this. So, the procedure will be same as for other conditions.
For eg suppose enrolled relation contains 1000 tuples and paid relation contains 500 tuples. Then distinct number of students must be atleast 500(because paid relation contains all distinct student and there might be some students in enrolled who haven't paid the fees yet).
Then In Query Ist, we need 500 comparison operations to select tuples where amount > x, and suppose the tuples where x > amount comes out to be 200. Then we get 200 * 1000 = 200,000 entries in output table from which we have to select only the tuples where enrolled.student = paid.student. So total amount of work = 200,000 + 500 = 200,500.
Now, if we go with Query IInd, we first need to do cross product = 500 * 1000 = 500,000, then filter comparison conditions on all 500,000 tuples. So total work done = 500,000 * 2 = 1,000,000
Clearly, we can see option C comes out to be right.
This example also counters the Option D, and hence option D is false.