Standard form of 25 is

Standard form of a natural number $N(>1)$ is $N={P_1}^{n_1}.{P_2}^{n_2}.{P_3}^{n_3}\mathrm{......}$

Where $P_1,P_2,P_3,\mathrm{...}$ are prime numbers and $n_1,n_2,n_3\mathrm{.....}$ are positive integers.

$25=5^1{.5}^1=5^2$  So option $\text{'}b\text{'}$ is correct.