Ncalculating the cost of common stock equity, the model which describes the relationship between the required return and the nondiversifiable risk of the firm is

 the model which describes the relationship between the required return and the nondiversifiable risk of the firm would be: The capital asset pricing model

The capital asset pricing model is a financial model that is used to determine the relation between risk and expected result.
In this model, we need to compared these following factors:
- Risk free rate
- Beta of the security
- expected market return