What is the angle which is $\frac{1}{3}$ of a right angle ?

Given angle is $90°$

To find : $\frac{1}{3}$ of a right angle 

A right angle is $90°$

$\frac{1}{3}$ of a right angle is $\frac{1}{3}\times 90°=30°$

Hence, the correct option is option B.