First Principles Thinking: one point at a time
B is correct. The CFA Curriculum procedure for key rate duration is to shift only the selected key point (for example, the 5-year rate) up and down, hold all other key rates fixed, reprice the bond, and then apply the duration formula.
Shifting the entire curve in parallel is the procedure for effective duration, not key rate duration.
Shifting the bond's yield-to-maturity is how yield-based approximate modified duration is computed, which is unrelated to key rate duration.