All are fine; the A's and B's are optional constructions. This is what you cannot do: I will bring a pen, pencil, or a notebook. Drop the pen, the pencil, and eraser.
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Gori(Q1) So...what you are saying is that I should place a definite/indefinite article either before the first noun only or before each of the nouns? Is my understanding correct?Yes.
Gori(Q2) Well, if this were true, do you mean I can also use the 1b), 2b), 3b), 4a), and 5a) even in formal writing?Yes.
Gor