Answer:
D. Is as vivid in its details of personal life as that gathered.
Explanation:
<em>Written in ink or engraved by stylus, more than 2,000 letters and documents on wooden tablets excavated at the site of the old roman fort at Vindolanda in northern England are yielding a historical account of the military garrison in the first and second centuries that </em><em><u>are so vivid in their details about personal life as they are</u></em><em> from Pompeii </em>
The sentence is dealing with comparisons of historical accounts and items from Vindolanda and Pompeii. The relative pronoun "that" is always related to the preceding noun/ noun phrase, in this case, the "historical account". So, the verb agreement will be singular. Thus, option D is the right answer.
Option A and B are wrong for it's use of plural 'are'.
Option C, for the use of 'their' which is also plural.
Option E is a wrong sentence construction 'as is that'.