The arc length is the same fraction of the circumference as the central angle is of $360^\text{o}$. Once you know the angle fraction, multiply it by the total circumference of 32.

You should get a simple fraction like $\tfrac{1}{4}$. Multiply that fraction by 32 to find the arc length.

In a new expression line, type (90/360)*32 or equivalently (1/4)*32 to represent the fraction of the circumference that matches a $90^\text{o}$ central angle.

Look at the value that Desmos displays for this expression; that number is the length of minor arc $\widehat{AB}$. Compare it to the answer choices.

A full circle has $360^\text{o}$. The problem says $\overline{OA}$ and $\overline{OB}$ form a right angle at the center, so the central angle $\angle AOB$ is $90^\text{o}$.

Find what fraction of the full circle this is:

So arc $\widehat{AB}$ represents $\tfrac{1}{4}$ of the entire circle.

The length of an arc is the same fraction of the circle’s circumference as its central angle is of $360^\text{o}$.

So, if the full circumference is 32 and the arc is $\tfrac{1}{4}$ of the circle, then the arc length is:

Now compute this product.

Calculate the product:

So the length of minor arc $\widehat{AB}$ is 8, which corresponds to choice B.