The problem asks us to find the image of the point $(a, b)$ when reflected in the $y$-axis.

1. Problem Description

The problem asks us to find the image of the point $(a, b)$ when reflected in the $y$-axis.

2. Solution Steps

When a point $(x, y)$ is reflected in the $y$-axis, the $y$-coordinate remains the same, but the $x$-coordinate changes its sign.

Therefore, the image of the point $(x, y)$ after reflection in the $y$-axis is $(-x, y)$.

Applying this to the given point $(a, b)$, the image of $(a, b)$ after reflection in the $y$-axis is $(-a, b)$.

3. Final Answer

The image of the point $(a, b)$ after reflection in the $y$-axis is $(-a, b)$.

So the answer is B. (-a,b).