How to decide for the plan (move 6)?

Hey guys, let me point you to the game lichess.org/cqcCMBog/black#11
It was shit and it was a blitz game, but what is interesting moment was at move 6.
As a black I have no idea how to play against these white formations.

My candidate plans were:
A) 6.- d5, closing the center, with c5 (hoping I can regain the pawn), or c6
B) 6. - d6 with c5.
C) 6.- b6 with c5 (or d5) - this seems to be in retrospect the best option if I want to do something with my c8 bishop, since Bg5 looks quite flaky because doesn't have enough space for retreat after h3.
All

How do you decide what do play? Any other plans you can think of?

Everything reasonable. Probably d6+e5 is a plan as well. Attacking the center with the levers e5 and c5 and trying to open up the position in the long run. Not sure how to start. Different day, different approach.

...d6 followed by ....c5 is a nice plan in such positions (why do we put the bishop on g7? To use it on the diagonal, so we try to open that diagonal)

Another thing to consider is that you can combine all the plans you mentioned. You can go ...d6, then ...d5 later AND ...e5, it depends on what white does. You just make sure to fight for the center and use that bishop.

I agree with A.

Closing the center, letting him gain space in the center usually hinders my development, so close it and block it before he starts advancing those central pawns!

...d6 and later ...c5. As white has traded his bishop for your knight, you Bf6 is very strong. As you have the bishop's pair you should try to open the position, not close it. You may even play ...Bg4 and Bxf3 so as to elimiate his Nf3 that defends the dark squares e5 and d4.
...d6 and ...e5 seems inappropriate as it restricts the activity of your Bf6 and allows him to slowe the position with d5, when his knights are stronger.
...d5 seems wrong as this also leads to a more closed position.
...b6, ...Bb7 and later ...c5 looks interesting. Kasparov has won some impressive games with a double fianchetto as white in the Scotch.

surprised nobody mentioned am immediate Nc6!? adding pressure to the center

Where is the pressure: e3 defends d4, so Nc6 exerts no pressure and only blocks the logical ...c5.

All plans seem reasonable and it might be a bit personal preference.

Personally I'd go with an immediate d5 which gains space in the center and the pawn cannot be challenged by c4 since white misplaced a knight on c3. I'd follow up (later) with c5, Nc6 and perhaps Bg4 and Qb6, putting pressure on the d4 (and b2) pawn. If white captures dxc5, you gain the long diagonal and the white pawn on c5 cannot be easily defended so you will gain it anyway.

My second choice would be the b6 plan you mention.

The other options (d6, c5 or d6, e5) lead to somewhat equal/symmetric positions. As white played rather passively (e3 etc), I think you should grab your chance and get active as black.