How this knight on C6 a hanging piece here?

Be6 is the hanging piece and f5 is the hanging pawn

But when I take the knight on c6, how does that take care of "sparing a hanging piece" recommendation? I mean how does taking c6 "unspare" the hanging piece?

I believe you are asked to play fxe6 instead

No miskit_mistake. That's precisely my confusion. As is written there, "the best move is bishop takes knight on c6". In fact in the review practice I tried taking bishop on e6, but Dr Wolf (it's a chess.com app) thought that wasn't the best move. 

the spared piece is e6, the pawn is f5, and they want bxc6+ then fxe6

it will

have patience

also bots mention stuff even though their best move doesnt even care, or does but later

also there is precisely 0 connection that says "the best move must take care of any issue if available"

Learn Chess with Dr. Wolf. It's a chess.com app

Possibly because of b5 c4 trapping the bishop.

After taking the bishop on e6, black replies with b5 and the bishop will be trapped

Oh yeah, that makes sense. Thanks a lot! As you can see I'm not even a beginner

Blunder---failure to BxN before fxB keeps white from going up a minor piece

BxN gives better compensation to white; B is spared from being trapped 

fxB converts a hanging pawn into a strongly placed pawn, restraining the B.  Follow with Ng6 & Nf7 with potential to gain another piece 

That's amazing! Wonder when I'll be able to think like that 

Chess.com tries again and again to implement a bot that can teach chess, and they are failing miserably. This "sparing a hanging piece" nonsense is just nonsense. Chess is way too complicated for a one-dimensional bot to understand it, let alone teach it.

(An aside for Centermass51075: "fxB" is not a valid chess notation under any system known to humankind. What is the point of writing barely legible stuff like that? Learn chess notation, please.)

Centermass51075's comments are contextual and I could perfectly understand them. Your comment will only discourage anyone who goes out of her/his way to help others.

Taking the knight forces a response from black that can result with the black light squared bishop being taken upon the next turn. If black responds to the knight capture with the pawn their pawn structure is also damaged, there is also the chance for black to blunder their queen. In that scenario they actually manage to save their bishop, but at what cost?