Doubt from isomers

Here is my method
is there any mistakes?

Are they pair of diasteroeomers?

Ya that's what I get(given answer also says Distereomers )
This method is correct na.

Can you guys show me how these(a,b) are not superimposable.

Pick let say a and put it on b you will find that Cl of a and H of b are coming over each other so non superimposable

After rotation they are superimposable .
@Achyut_2020

How can you please tell? As both have different configuration 2S,3S and 2R,3R

They aren't, there might be an error in your rotation...

I dunno, I have done it by R and S...
If this method is giving correct answers in other questions too then it is correct....

Can you send the pic after rotating b as they have did @Abhishek_2020_5
I think I did something wrong but couldn't find