From the given information, we know that there are 6 books placed side by side, and we can categorize them based on their cover color and type of book. Let`s represent blue cover with B and red cover with R, and law reports with L and Gazetteers with G.

So, we can represent the arrangement of the books as:

BBBLLL - where B represents blue covered books, L represents law reports, and there are three of each

Now, we know that only D and F are new books, so the arrangement becomes:

BBBLLN

where N represents the new books, which are D and F.

We have to find a new book that is a law report with a red color. From the given information, we know that there is only one new law report, which is D. And since D is a new book, it must have a red cover because all the new books have red covers. Therefore, the answer is D.