Opp is new to the table.

PokerStars No-Limit Hold'em, $0.25 BB (9 handed) - Poker-Stars Converter Tool from FlopTurnRiver.com

MP2 ($24.90)
MP3 ($4.10)
CO ($11.60)
Button ($17.15)
Hero (SB) ($24.20)
BB ($31.85)
UTG ($5)
UTG+1 ($29.35)
MP1 ($10.10)

Preflop: Hero is SB with 5, A
6 folds, Button raises to $0.50, Hero calls $0.40, 1 fold

Flop: ($1.25) K, A, 5 (2 players)
Hero bets $1, Button raises to $2.50, Hero raises to $5.50, Button raises to $8.50,
Hero?