How can we define optical isomerism?What is the example of non enatiomeric optical isomers?
There are, in the sense you are asking the question, three types of optical isomers: +, -, and optically inactive. This is detailed in pages of the textbook you have uploaded.
You should think about optical isomers in how they are determined. Imagine you have a machine which we will call and &quot;optical isometer&quot;. To use this machine, you put in any compound in solution, and it will tell you one of three outputs:

&quot;This compound polarises light in the clockwise direction&quot;
&quot;This compound polarises light in the anti-clockwise direction&quot;
&quot;This compound does not polarise light&quot;

Now, our optical isometer doesn't know any chemistry, but it does know how to tell if light is being polarised by our compound in solution. If you fundamentally think of isomers as identical in their molecular formula, but differing in their spatial arrangement of atoms, you need only imagine as optical isomers being different in how the spatial arrangement of atoms polarises light.
Let's imagine we have a solution of a tartaric acid, but we don't know what isomeric form it is in. The options are:

meso-tartaric acid
(+)tartaric acid
(-)tartaric acid
A mixture of (+) and (-)tartaric acid.

Where I think you have been confused by the textbook is that option number 4 doesn't actually describe different spatial arrangements of tartaric acid, rather it describes a mixture of options 2 and 3. So when it says there are four isomeric forms, it isn't strictly correct about that.
Now, how will we determine what isomer of tartaric acid we have in solution? One thing that we can do to find this out would be to use our optical isometer. We put our solution in and it outputs: &quot;This compound polarises light in the clockwise direction&quot;. We know that of our four possible options, only (+)tartaric acid polarises light in the clockwise direction. Likewise, we would know it to be (-)tartaric acid if the light were polarised in the anti-clockwise direction.
What if our optical isometer output, &quot;This compound does not polarise light&quot;? Then this means that our compound is optically inactive. We know that both the meso- form and the racemic (mixture of (+) and (-)) don't polarise light in either the clockwise or anti-clockwise direction, and so we would need further analysis to find out for sure!
There are other types of light polarisation, but not in the sense this book is talking about optical isomers. This textbook seems to be a little confusing, but these ideas are actually really well explained on places like wikipedia or khan academy, so next time you get stuck, don't worry too much about the textbook but give other resources a shot!
Hope this helped,
Edan