who (rating) can spot the correct move here?

Black to Move.

This isnt about why i won or lost. I went to review this game after it ended, and i was surprised to see a fork as a mistake - if i were to be sure of ANY move i made during that match, it was that fork (talking about d4 obv) - but computer says no. 

I was curious about optimal play and i did play around with the position to understand why. but, im curious as to how high a rating do you need in order to see this without the pc.

(im curious who can come up with the answer -without an engine- and what their rating is)

Make it a pgn

It’s not immediately obvious why it’s a bad move. Even when you notice they can play Bxb7, attacking your (defended) Rook with their Bishop, while you are still attacking two minor pieces with the fork, that’s a trading sequence that’s usually beneficial. In order to notice why the move d4 is bad, you have to notice the follow-up threat Qh5+ as well. That's not at all easy, depending on your rating. It’s impossible to say at what rating players will see that d4 is a bad move in this specific position.

My perspective is that higher rated players would be more aware of the dangers and threats that accompany positions like this. Dan Heisman teaches the principle "Don't start a fight until your king is safe!!" (especially if your opponent's king is already safe) or "Don't start a fight in the center if you are not castled; especially if the opponent is castled.".

While your opponent has already castled to safety, you did practically the opposite. After you have moved away all pawns adjacent to the King, your King has almost no protection. Your King is even vulnerable to attacks from both diagonals. It would take at least a couple of moves to get your King in a safer position.

The difference between players <1000 and >1000 is not whether they see why d4 is a bad fork. It’s in the way they play the three moves before that (9. f6 10 fxe5 and 11. Nx5). Two moves are pawn moves and the third is moving a piece that’s already been developed (your only developed piece even!), while the other Bishop, the Knight, the Queen and two Rooks are not yet developed. You are engaging in a fight with less than half your army against an active and fully deployed army. And the King isn’t even safe yet.

D4 immediately isnt terrible, but bxb7 is annoying, so nxb first jumps out. (Bf4 instead of bxb7 doesnt work, since you could just take the knight.) So exchange first, then d4 and white can delay by moving a rook to the file, but so what, black's queen moves and wins the piece next move.

I'm around 17-1800, but I dont think you need to be nearly that strong to reason this through. It does seem a bit much for a 900 to figure out during a game, but 12-1300 should be able to get it.

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I would not play the fork due to lack of king safety. I win a piece for a pawn but don’t see my follow up and do see lots of open files and diagonals for white.
I play …Nf6 hoping to castle soon.
(1500-ish here).

What i saw is d5 Bf4 Nxf3+ Qxf3 dxc3, my rating is 700+, tell me if my chess language is right.