$7^{5x} = 243 = 3^5$ $\implies$ $7^x = 3$, então $x = \log_7 {3}$. Portanto $\large{7^{-3x} = 7^{\log_7 {\frac{1}{27}}} = \frac{1}{27}}$ Alternativa $\mathbb{(C)}$