The denominator in $A$ is composed of only $2$'s, $(256=2^8)$. 

The denominator in $B$ is composed of only $2$'s and $5$'s, $(100=2^2\times 5^2)$. 

In fully reduced form, the fraction in $D$ is equal to ${\displaystyle \frac{7}{20}}$ and the denominator $20$ is composed of only $2$'s and $5$'s $(20=2^2\times 5)$.

By contrast, the denominator in $C$ has prime factors other than $2$'s and $5$'s $(27=3^3)$, and in fully reduced form, the fraction in $E$ is equal to ${\displaystyle \frac{1}{15}}$, and $15$ has a prime factor other than $2$'s and $5$'s $(15=3\times 5)$.

Therefore, options $A$, $B$ and $D$ are the correct answers.