Dear A Z:

You wrote:
> a. He bought the red or blue notebooks.
> (He bought both the red and the blue notebooks.)
>
> Is a is correct?
Not if you mean he bought both kinds. “Or” means “one or the other,” not both.
>
> b. He bought red or blue notebooks.
> c. He bought red and blue notebooks.
>
> Is there a difference between b and c?
Absolutely. B means that he bought one kind or the other. It would normally be said by someone who could not remember precisely or did not know for sure. He bought either all red ones, or he bought all blue ones; he did not
do both.

C means that he bought both kinds of notebooks, some were red, some were blue.
>
> It seems to me that one would not use b if one were speaking of one
> occasion in which he has bought both red and blue notebooks.
>
That is true.

In English, and throughout the Western world, this distinction is very important. So much of Western writing and communication is based on Aristotelian logic in which the difference between AND and OR can be critical.