Defending K9o pre is fine, but folding to the tiny flop c-bet with ~46% equity and getting ~3.7:1 is a bad mistake—either raise as a semi-bluff or call here.
Flop comes Th Td 4d, paired and semi-wet. BU has the range advantage here, so checking our whole range, including K9o, is the standard approach. We don’t want to lead into a strong hand like TT, T9, or overpairs with just K-high and a backdoor diamond draw.
BU fires 1.7BB into 4.5BB, giving us about 3.7:1 and needing only ~21% equity. K9o has roughly 46.8% equity versus BU’s c-betting range, so folding is a big mistake. This hand is a strong candidate to raise as a semi-bluff, using our backdoor diamond draw and BU’s wide c-bet range to pressure their weaker holdings. Calling is also fine; folding just gives up too much equity.
Note: Folding K9o to the small flop c-bet with ~46% equity and getting ~3.7:1 is a big mistake; this hand should either raise as a semi-bluff or call, not fold.