There are two types of articles, definite and indefinite.

Definite article, the, is used to describe a specific item: "I don't want just any chair, I want the chair!"

Indefinite articles, a and an, are used to describe any item of the same property: "Just find me a chair, I need to sit down and take a break."

"the" chair in the first example is referring to a specific chair that both the speaker and the listener are presumed to know.

"a" chair in the second example is simply referring to any chair.

I do recall seeing a fair number of these articles being the deciding factor in some tougher SC questions. I tried to search for them and didn't have any luck finding one.

I hope I was clear in the explanation. I am not as good at explaining SC as before.

Thanks uwhusky but unfortunately i was looking from SC perspective, how the subtle idfeerence is created by using the article in options

Ya I figured that to be the case at end of my reply...=)

I'll see if I can dig up an example, and I'll let you know if I do.

Without examples it's harder to say, but you should expect the definite article "the" when referring to a specific thing. You should also expect "the" with superlative expressions (the best/wettest/most swollen/least expensive et cetera). In comparative expressions, you can ask for a better seat, or the better seat if there is only one better, regardless of whether there is a "than . . ." following it.