Researchers have found...

Stacey Koprince wrote:

so this means, for "comma(,) and" there is no rule as such in gmat
it can be used anywhere

It's basically a one-way rule. If you are going to connect two independent clauses using "and" then you actually need "comma and."

But there are other circumstances in which you could have "comma and." For example:

She bought milk, bread, and cheese at the store. (list of three things)

She bought milk at the store, which just re-opened yesterday after a fire, and played tennis at the club. (the stuff after the "and" is not an independent clause, but the comma is necessary because it is enclosing the "which" modifying clause)

Stacey Koprince wrote:

the soft copy called Manhattan Review - sentence correction guide..is what i have downloaded from Manhattan website only.

Okay, you have something from Manhattan Review, which is a different company. FYI, when everyone here talks about the ManhattanGMAT Sentence Correction Guide or the Manhattan SC Guide, the Manhattan Review book is not the book they're talking about. The two companies are different companies and the two books are different books.