[tex]\sf \left(\dfrac{1}{7}\right)^3\ is\ basically\ like\ saying\ you're\ multiplying\ \dfrac{1}{7}\ by\ \dfrac{1}{7},\ three\ times.[/tex]
[tex]\implies\ \sf \dfrac{1}{7}\ \times\ \dfrac{1}{7}\ \times\ \dfrac{1}{7}\\\\\\\implies\ \dfrac{1}{343}[/tex]
Therefore, (1/7)³ = 1/343, implying that the answer is option (D).
Hope it helps. :)
