For those who are expert in queens gambit line, I wanna ask you an help...in this position in the diagram white has a problem in his c1Bishop...it is hard to activate and black pieces looks more active than white, it is true that white controls the center and have very well placed pieces except for c1Bishop but that c1Bishop weakens white...how should white continue this game/position or is there any other good lines here to be recomended or even other variation of Nf3/d4 opening...Tnx

Why a4? The bishop will come out once the game opens up, until then it could play a defensive role.

a4 prevents the pawn on b7-b5 which more favors black if black defended corectly...

to activate that bishop is really hard to do, the only way I think is to sacrifice a pawn...

@t1998

Sometimes being patient is important in chess.

Later on, after castling and after moving away their f3-knight, White will be able to play f3 followed by e4 which will free White's queenside bishop.

Also, remember that in most chess positions there is always more than only one relevant positional detail!

White's queenside bishop is only one of many.

E.g. White's solid space advantage in the center outweighs this issue with the seemingly passive bishop.

Also, it's not even clear whether Black's decision to develop their queenside bishop to f5  prematurely was a good plan (although this seems to be a Slav main line), because White may have a chance to exchange it for a knight (it can't retreat!), and then White will have the longterm advantage of the bishop pair.

And White will manage to open up the position for their bishops sooner or later, a seemingly good plan of how to do this could be f2-f3 followed by e3-e4, and then go for the breakthrough d4-d5.

If until then White managed to get the bishop pair advantage, then the position will magically become very unpleasant for Black.

A possible maneuvre for White to grab Black's f5-bishop could be Nc3-e2-g3. The f3-knight should keep the e5-square under pressure (maybe it gets an opportunity to hop there under favourable circumstances).

Well, this would be my spontanous longterm battle plan for White, whether it's any good or too optimistic ... good question. Maybe you can find some games about this variation where they played the position with this plan, and see how well it worked for White.