A long put butterfly spread is a combination of a short put spread and a long put spread, with the spreads converging at strike B.
Ideally, you want the puts with strikes A and B to expire worthless, while capturing the intrinsic value of the in-the-money put with strike C.
Because you’re selling two options with strike B, butterflies are a relatively low-cost strategy. So the risk vs. reward can be tempting. However, the odds of hitting the sweet spot are fairly low.
Constructing your butterfly spread with strike B slightly in-the-money or slightly out-of-the-money may make it a bit less expensive to run. This will put a directional bias on the trade. If strike B is higher than the stock price, this would be considered a bullish trade. If strike B is below the stock price, it would be a bearish trade. (But for simplicity’s sake, if bullish, calls would usually be used to construct the spread.)