First Principles Thinking: core idea
A is correct. The CFA Curriculum defines Macaulay duration as the present value weighted average time to receipt of cash flows. Once each weight is computed, the duration is the sum of each time to receipt multiplied by its weight. This produces a time measure in periods, later annualized if needed. Therefore choice A is the correct expression.
Why top distractor is wrong (PDF-based misconception): B weights cash flow amounts rather than the timing of those cash flows, so it does not produce a duration measure.
Why remaining distractor is wrong: C omits dividing by bond full price, which is already built into the weight and is necessary for the weighted-average formula.