$a){\left(\frac{7}{8}\right)}^2=\frac{7}{8}\cdot \frac{7}{8}=\frac{49}{64}$  

$b){\left(\frac{2}{3}\right)}^4=\frac{2}{3}\cdot \frac{2}{3}\cdot \frac{2}{3}\cdot \frac{2}{3}=\frac{16}{81}$  

$c){\left(-\frac{5}{6}\right)}^2=\left(-\frac{5}{6}\right)\cdot \left(-\frac{5}{6}\right)=\frac{25}{36}$  

$d){\left(-\frac{1}{2}\right)}^3=\left(-\frac{1}{2}\right)\cdot \left(-\frac{1}{2}\right)\cdot \left(-\frac{1}{2}\right)=-\frac{1}{8}$  

$e){\left(-\frac{3}{4}\right)}^4=\left(-\frac{3}{4}\right)\cdot \left(-\frac{3}{4}\right)\cdot \left(-\frac{3}{4}\right)\cdot \left(-\frac{3}{4}\right)=\frac{81}{256}$  

$f){\left(-\frac{2}{5}\right)}^3=\left(-\frac{2}{5}\right)\cdot \left(-\frac{2}{5}\right)\cdot \left(-\frac{2}{5}\right)=-\frac{8}{125}$