First Principles Thinking: yield-based duration requires a defined yield-to-maturity
A is correct. Modified duration is a yield-based statistic that presumes a single, well-defined yield-to-maturity computed from a known stream of future cash flows. When a bond contains an embedded option, exercise of that option, and therefore the timing and amount of cash flows, depends on where market interest rates settle relative to the coupon. Cash flows are no longer certain, so a single yield-to-maturity cannot be reliably defined. The CFA Curriculum therefore prescribes effective duration, a curve-based measure that captures price sensitivity to a parallel shift in the benchmark yield curve, with prices generated by an option-pricing model. Reason R supplies precisely the mechanism that makes modified duration unsuitable, so R is the correct explanation of A.
Option B is incorrect because the link between the assertion and the reason is direct: the absence of a well-defined yield-to-maturity is the very reason a yield-based measure cannot be used.
Options C and D are incorrect because both statements are true: effective duration is the appropriate measure for option-embedded bonds, and the cash-flow uncertainty arising from option exercise is real.