ISSUE:

Hello.
In the image that I have attached I have to say which compounds are optically inactive. It is my understanding that optically inactive compounds are achiral and therefore have a plane of symmetry. In the solution it comes as it is 1 2 4 5 6 8.
I don't understand why the molecules 1 2 5 6 and 8 are achiral... because I don't see that they have a plane of symmetry...

I would say that it is because of this, I am attaching a file.

Hello, in the drawing you have it well explained.
The 1 is achiral as it has a plane of symmetry, to see it you must rotate the molecule and place the -OH towards the same side. The 2 does not have asymmetric carbon.
To be chiral, a substance needs at least one asymmetric carbon. The 4 has a plane that passes between the carbons that have the -OH
The 6 lacks asymmetric carbons.