(Britain,colloquial) That is the right person or thing; that's it.
2006, Jennifer Kirk, Countdown to Extinction
"Pass the block of gear across will you? It's there on the table beside you." He looked round, and spied a generous block of dark brown resin wrapped in shrink wrap. Where the plastic had been peeled back, a hunk had already been removed, revealing the glassy green inside of the block. "That's the bunny," Rob was saying, "pass the whole lot across on the board."
2007, Kate Johnson, Ugley Business
"I talked very nicely to an Irishman." "The one in the cell?" "That's the bunny."