What is the base of exponent 4?

a) [[$ \left(-\dfrac{5}{6} \right)^4 $]]
The base is

b) [[$ \dfrac{2^4}{3^3} $]]
The base is

c) [[$ -2\dfrac{1^4}{4} $]]
The base is

d) [[$ -\left( \dfrac{2}{5} -x \right)^4 $]]
The base is