Merger Arbitrage - Payoff

Can someone tell me why the payoff of a merger arbitrage resembles a riskless bond + a short put option.

I just did a problem in the book where they said to calculate the payoff of a merger if it did well and if it failed.

So if they did well…they made profit.However, when they failed, they had a loss. So how does it even resemble a riskless bond + short put option?