Toynbee was a scholar of Greece and Rome, with a preference for the

TOYNBEE ANSWER

OFFICIAL EXPLANATION


THE PROMPT
• The first thing that should jump out at you is the jarring verb are.
See my analysis of option A, below.

• strategy lesson
The sentence is nonsensical. I would not—and did not—waste time trying to figure out from the prompt what the sentence was trying to say.
I moved to option B, which was not much help.
Option C let me know that this sentence is testing noun clauses. Noun clauses are singular.
I write about noun clauses in this post, here.

This question offers us a good strategy lesson: when the prompt is awful but you are not sure why, do not waste time trying to figure out what to do with option A.

Ready? Keep that option, but do so tentatively and be looking for an option that makes things clearer and/or allows you to pinpoint the basis of error in the options you keep.

Many of you are taught to understand the meaning of the sentence before you start.
I often do not agree with that approach.
If need be, I use all five options to understand meaning and to spot issues.

If I cannot eliminate an answer based on a clear error by the time I have finished reading it, I keep that option.
Try giving yourself three seconds to decide whether to eliminate an option.
If you cannot eliminate the answer, keep it and move on.
We are not looking for the best answer.
We are looking for the four worst answers.

Very often, reading through the options gives us insight into what the sentence should say and which answers fail to express its meaning.
If you do not like my advice and feel more comfortable trying to sort out meaning from the prompt, ignore what I am saying.

• Meaning?
The fact that Toynbee was a scholar of Greece and Room, and a close student of Byzantium and of the modern Balkans is apparent on every page of his last book.

• Issue tested: noun clauses and the fact that they are singular.

• what the heck is the subject? The modern Balkans? No.
→ The phrase the modern Balkans is the object of the preposition of.
→ RULE: The object of a preposition is never the subject of a sentence.
Wrong: Of the modern Balkans are apparent on every page.
• No subject exists for the verb are
• The problem at the moment is the lack of subject, not the verb are. (The verb are becomes a problem.)
ELIMINATE A

• be wary of being.
• same problem as that in A: no subject exists for the to be verb are
ELIMINATE B

• Aha. Suddenly the word that appears.
When "that" shows up at the beginning of the sentence, it often means the fact that.
• Now we have a noun clause:
[the fact] That Toynbee was a scholar of Greece and Rome, with the preference for the former, and a close student of Byzantium and of the modern Balkans
• that whole thing, the noun clause, is the subject of the sentence. And that whole thing is singular.
• the verb are is plural. Subject-verb disagreement.
ELIMINATE C

• looks good: in contrast to A and B, (D) contains a subject. In contrast to (C), option D gets the verb correct.
→ Noun phrases are always singular. They take the verb is
KEEP
• what does which refer to? No antecedent exists. What is apparent on every page?
• If in doubt, compare to (D). What is apparent on every page? [The fact] that Toynbee was a certain kind of scholar.
No contest. D wins.

The correct answer is D.

Notes

Everyone who posted had exactly the right idea or instinct about each option, but as far as I can tell, no one mentioned noun clauses, which can also be called nominal clauses or substantive clauses. I don't care whether you know the jargon.
I do care whether you understand that a long clause—itself with a subject and verb!—can be the subject of a sentence (or the object of a verb, or other things).

Let's just formalize what you all cleverly saw:
Something basic is wrong with options A, B, and E..
Options A and B explicitly lack subjects.
Option E's which contains no antecedent and so lands us in the same place as A and B.

Furthermore, noun phrases as subjects are singular—and they are more common on the GMAT than you might realize.
Noun phrase as subject: How people respond to failure or mistakes is a good measure of character.
Noun phrases can be used as objects of a verb, this way: I saw who stole the cookies.

COMMENTS
bm2201 and Iceman2020 , welcome to SC Butler.

I apologize for the tardiness of this answer. I'm moving to a new place. I do not advise trying to move in the middle of a pandemic.
Reported numbers in my part of the U.S. are rising rapidly. Thanks for your patience. I will catch up.

These answers range from good to very good. Nice work. Kudos to all.