Đáp án đúng: D
Phương pháp giải:
Giải bài toán ngược để tìm \(x\).Giải chi tiết:\(\begin{array}{l}\dfrac{1}{3}:\left( {2x - 1} \right) = \dfrac{{ - 4}}{{21}}\\\,\,\,\,\,\,\,\,\,\,2x - 1 = \dfrac{1}{3}:\left( {\dfrac{{ - 4}}{{21}}} \right)\\\,\,\,\,\,\,\,\,\,\,2x - 1 = \dfrac{{ - 7}}{4}\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,2x = \dfrac{{ - 7}}{4} + 1\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,2x = \dfrac{{ - 3}}{4}\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x = \dfrac{{ - 3}}{4}:2\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x = \dfrac{{ - 3}}{4} \cdot \dfrac{1}{2}\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x = \dfrac{{ - 3}}{8}\end{array}\)
Vậy \(x = \dfrac{{ - 3}}{8}\).
Chọn D.