A. if you found a stock with a zero historical beta and held it as the only stock in your portfolio, you would by definition have a riskless portfolio.
b. the beta of a portfolio of stocks is always larger than the betas of any of the individual stocks.
c. the beta of a portfolio of stocks is always smaller than the betas of any of the individual stocks.
d. it is theoretically possible for a stock to have a beta of 1.0. if a stock did have a beta of 1.0, then, at least in theory, its required rate of return would be equal to the risk-free (default-free) rate of return, rrf.