$$ Select the correct mirror image of the given figure when the mirror is placed at MN, as shown.

To find the correct mirror image of the given figure when the mirror is placed vertically on the right side at line MN, we reflect the elements horizontally (left becomes right and right becomes left):

1. Diagonal Line:
In the original figure, a diagonal line goes from the top-left corner to the bottom-right corner. When reflected horizontally across mirror MN, this line will go from the top-right corner to the bottom-left corner.

2. Triangle Element:
In the original figure, an upright triangle ($\text{Δ}$) is located in the upper-right section (above the diagonal line). In the mirror image, it will move to the upper-left section, while remaining upright.

3. Arrow Element:
In the original figure, an arrow pointing towards the top-right direction originates from the bottom-left corner (below the diagonal line). In the mirror image, it will originate from the bottom-right corner and point towards the top-left direction.

Comparing these transformed features with the given options, the correct mirror image corresponds to the fourth option: