(B) is the correct option!
As, SQL is a row by row processing of the relation data, it takes one row apply both the conditions
where $columnB>3000$ and $ColumnC<3000$,
And, if the record passes both the conditions then it will be selected. if there are n records in a table then we need to fetch exactly them once.
But if we first find those records where $columnB>3000$ and then we find the records where $ColumnC<3000$, and then take their INTERSECTION, then it's a too much work, as, first we are fetching the entire table twice and then applying INTERSECTION operation, furthermore, intersection is an expensive operation because it has to compare the results set and then it removes duplicate also.