问题如下：
An analyst is most likely to use returns-based attribution when:
选项：
A. the portfolio has a low turnover.
B. the holdings for the portfolio are not available.
C. she wants the analysis to be as accurate as possible.
解释：
B is correct. Returns-based attribution is typically used when the holdings data are not available. Neither A nor C is correct because returns-based attribution is the least accurate of the three approaches.
