Which one is supposed to be a defensive castling ?? king side or queen side ??Both has some advantages and disadvantages........I need ur opinion plz

Both sides are defensive castling. However, I believe that your king will in general be in a slightly safer posistion if you castle king side. However, that's a very general rule. Queen side castling has the added benefit of placing a rook on the d file, which is a center file and a generally good place for your rook to be. Really though, it all depends on the circimstances when you castle. Of course, let's just wait for someone better than me to disprove everything I just said.

O-O: only requires two piece moves; the king is generally a bit safer (no open diagonal to the king); all pawns are defended by the king; the rook is not placed on an open file.

O-O-O:  requires three piece moves; the king is a bit more open; the a-pawn is not defended by the king; the rook is placed on an open file.

Both O-O and O-O-O protect your king more than leaving him in the center, but due to the fact that O-O-O generally offers a bit less protection, requires more determination to execute (as it requires a queen move), and places the rook on a central file immediately, I would say that O-O-O is generally a bit more offensive than O-O.  But it always depends on the position.  I think a bigger question is why you are asking this question? Perhaps I'm not getting it, but it doesn't seem to have much importance as a general question.