To determine the correct mirror image of a figure when a mirror is placed vertically on a line MN, we must understand how each element of the figure reflects across the mirror.

Here are the steps to solve the problem:

1. Identify the line of reflection, which is the mirror line MN.
2. Observe each component of the figure and consider how its position changes with respect to the mirror line. For vertical reflection, any point and its mirror image are equidistant from the mirror line, but on opposite sides.
3. The shape and orientation of the components remain the same, however, their positions relative to the mirror line become reversed.
4. Check each provided option to see which figure accurately represents these transformations.

After evaluating all options, the correct mirror image of the original figure when the mirror is at MN is: