This is a small underline question, thus we can try to prephrase and if that does not work find errors and eliminate based on them.
Hmmm ......I get a sense that the way Germania is categorized is amusing, but am not able to prephrase something. So I do not waste time and move to the choices.
A - interesting, choice A is different from the underlined section which will never happen on a real GMAT question. Anyways focusing on the question, the underlined section has a shift in meaning - looks like the critics are amused by the scholars, This is not the case. Let us eliminate all choices that have this issue.
Choices A, C and E out.
Now, B and D are left.
B - 'as if' is not the correct construction here. It was categorized as an ethnographic treatise not as if one.
D - Correct choice, the critics were amused by "how the scholars categorized Germania which was written by Tacitus"
Hence pick D