In the context of a plane mirror, each option refers to a different property of the image formed:

1. `It is of the same size as the object` is true - a plane mirror does not magnify or reduce the size of the image.

2. `It is laterally inverted` is also true - something moving right in reality appears to move left in the reflection.

3. `It is a real image` is the correct answer because it is not true. A real image is formed where light from an object actually comes together or converges, and can be projected onto a screen. But the image formed by a plane mirror appears to be where the light is not actually present, it is virtual not real.

4. `It is formed as far behind the mirror as the object is in front` is true - this is why the image in a mirror seems to be "inside" the mirror at the same distance as the object is from it.

In conclusion, the third option is not true about the image formed by a plane mirror.