### Theory:

Reflection symmetry:
A pre-image reflecting itself as an image about a line is called reflection symmetry.
The line reflecting the image is called the line of reflection.

Have a closer look at the following examples.

Here, the yellow line is the line of reflection.

Whatever portion placed on one side of the line of reflection gets reflected on the other side of  the line of reflection.

In other words, the pre-image is flipped to an image about the line of reflection.

Example:
$$1$$.

Let us analyze the given example in detail.

1. The object with the blue outline is the pre-image, and the one in pink is the image.

2. The image of $$A$$, $$B$$ and $$C$$ is $$A'$$, $$B'$$ and $$C'$$ respectively.

$$2$$.

Let us analyze the given example in detail.

1. The object with the blue outline is the pre-image, and the one in pink is the image.

2. The image of $$A$$, $$B$$, $$C$$ and $$D$$ is $$A'$$, $$B'$$, $$C'$$ and $$D'$$ respectively.