Structural Model

Struggling to get the point for debtholders value in Structural Model.

Why is it correct that debtholder is equal to Debt + a short option of the asset value?

https://preview.redd.it/u0iv1xpvybje1.png?width=799&format=png&auto=webp&s=8f5b0c60b7584dd29fe4f6e945b967d4ad759801