Which of the following best describes Treynor's Ratio?

a$.$ It measures the excess return per unit of total risk, where total risk is measured by the portfolio's standard deviation.

b$.$ It measures the excess return per unit of systematic risk, where systematic risk is measured by the portfolio's beta.

c$.$ It measures the excess return per unit of unsystematic risk, where unsystematic risk is measured by the portfolio's variance.

d$.$ It measures the absolute return of the portfolio over the risk$-$free rate