A portfolio manager notices that her fund's performance closely tracks the market index, but with slightly lower volatility. Which of the following statements is most likely true about this portfolio?

Select one:

a$.$ The portfolio consists entirely of risk$-$free assets

b$.$ The portfolio's beta is approximately $0\mathrm{.}9$

c$.$ The portfolio's standard deviation is equal to its beta

d$.$ The portfolio has eliminated all unsystematic risk