$a)$

$0,5=\frac{5}{10}=\frac{1}{2}$

$0,75=\frac{75}{100}=\frac{15}{20}=\frac{3}{4}$

$0,125=\frac{125}{1000}=\frac{25}{200}=\frac{5}{40}=\frac{1}{8}$

$0,375=\frac{375}{1000}=3\cdot \frac{125}{1000}=3\cdot \frac{1}{8}=\frac{3}{8}$