$a\text{)}\sqrt[3]{27\cdot 125}=\sqrt[3]{27}\cdot \sqrt[3]{125}=3\cdot 5=15$    

$b\text{)}\sqrt[4]{81\cdot 256}=\sqrt[4]{81}\cdot \sqrt[4]{256}=3\cdot 4=12$   

$c\text{)}\sqrt[3]{-\frac{27}{64}}=-\frac{\sqrt[3]{27}}{\sqrt[3]{64}}=-\frac{3}{4}$  

$d\text{)}\sqrt[4]{\frac{625}{256}}=\frac{\sqrt[4]{625}}{\sqrt[4]{256}}=\frac{5}{4}$