First Principles Thinking: kinked CML with a higher borrowing rate
B is correct. The CFA Curriculum shows that once the investor borrows, the relevant financing rate is the borrowing rate $R_b$, not the risk-free lending rate $R_f$. The line to the right of the market portfolio therefore has slope $[E(R_m)-R_b]/\sigma_m$, which is smaller if borrowing costs more than lending. That creates the kinked CML. So the borrowing side uses $R_b$ in the slope.
A is incorrect. This is the lending-side slope between the risk-free point and the market portfolio, not the borrowing-side slope.
C is incorrect. The CML slope is based on expected market return less the relevant financing rate, not the spread between the two financing rates.