In which scenario would you use the ROLLUP operator for expression or columns within a GROUP BY clause?

[answerhappygod]

a) To find the groups forming the subtotal in a row
b) To create group-wise grand totals for the groups specified within a GROUP BY clause
c) To create a grouping for expressions or columns specified within a GROUP BY clause in one direction, from right to left for calculating the subtotals
d) To create a grouping for expressions or columns specified within a GROUP BY clause in all possible directions, which is cross-tabular report for calculating the subtotals