What's black's best move?

::: spoiler Hint 1

Black must avoid checkmate. :::

::: spoiler Hint 2 (a very big hint)

Black's e pawn. :::

::: spoiler Spoiler https://www.chess.com/analysis/game/live/122884427351?tab=analysis&move=72

e1=N+ - underpromotion!

Underpromotion is very rare. The key thing is it's a check and a check is a tempo which is a very important concept in chess. Why not a queen? No tempo :( so white has checkmate in 3 starting with Rc8+.

I watched this speed chess game live and the move happened even before I thought about underpromotion. I wonder how long it would have taken me to see it in a slow game.

:::