The answer should be D.

That < a fact> is, apparent <>

I just don't seem to get why C should be selected.

In this case, we are having two facts. That X and Y.

I was assuming it to have a plural verb.

Option A; Toynbee was a scholar of Greece and Rome, with a preference for the former, and a close student of Byzantium and of the modern Balkans are' this statement does dont give us and insight of what is to come,rather it gave a past tense picture of an act in the past and a dead man.

Option B; This is the correct answer 'Toynbee being a scholar of Greece and Rome, with a preference for the former, and a close student of Byzantium and of the modern Balkans are ' considering the fact that 'being' is present it shows what Toynbee is.

Option C; That Toynbee was a scholar of Greece and Rome, with a preference for the former, and a close student of Byzantium and of the modern Balkans are' the use of 'that. does not give an insight to what is going to happen in the future in the book.

Option D; 'That Toynbee was a scholar of Greece and Rome, with a preference for the former, and a close student of Byzantium and of the modern Balkans is' this option is incorrect because of the use of 'is' there are two elements that requires a plural verb which are 'student of Byzantium and of the modern Balkans' these are two groups that must attracts a plural verb.

Option E; Toynbee, a scholar of Greece and Rome with a preference for the former, and close student of Byzantium and of the modern Balkans, which is' this is incorrect because it follows the same mistakes as stated for option D above.