Hi sucheta.lahiri,

This is an example of a Quantitative Comparison question from the GRE, so it's out-of-place here on a GMAT website. You might be better served by seeking out GRE-specific websites to ask your questions.

That having been said, the concept involved is a rarer mathematical concept that has to do with how the products of numbers behave. For example, watch what happens as two numbers get closer and closer together:

(1)(9) = 9
(2)(8) = 16
(3)(7) = 21
(4)(6) = 24
(5)(5) = 25

If the two numbers increase/decrease by the same amount, then the product gets bigger (and it's biggest when the two numbers are equal). The same principal applies in this QC question (and to rectangles and squares in general).

GMAT assassins aren't born, they're made,
Rich