Type: Assumption Fill-In-The-Blank (odd variation of an assumption question)
BID (Boil It Down): Describes events in 1165, and father alive, but died in 1167 -> Written between 1165-1167
Missing Information: No other factors/variables
Goal: Even though this a fill in the blank question, it actually asking for something the reasoning assumes. Find an option that the argument depends on. One thing that stands out as assumed is that the accounts are accurate and can be trusted.

(A) the life of Prince Sviatov is not the subject of any other medieval Moringian texts
The argument stands whether the prince is the subject of other works or not, so boot this option. It's not required for the argument that Sviatovin was written between 1165-1167.

(B) the author of Sviatovin intended it to provide as accurate a report about Prince Sviatov's exploits as possible
The argument that the book was written between those dates does NOT rely on the assumption about the work's INTENT. The work could have a different intent than to be as accurate as possible, and still provide an accurate account of Sviatov's family, and the timeline.

(C) the diagram accurately represents the composition of Sviatov's family at the time Sviatovin was written
Yes! The argument completely depends on the assumption that the family tree accurately represents the composition at the time because what if it didn't? Then we lose the 1167 end of the range used to back up the whole theory that the work was written between 1165 and 1167. If C weren't true, then the work could have been written way out into 1175 for all we know.

(D) Sviatovin is the earliest Moringian text whose composition can be dated to within a few years
The argument in NO way depends on the assumption that this particular book was the EARLIEST that can be dated to within a few years. Whether or not that's true, the argument can still stand, so D is not something the argument depends on.

(E) Sviatovin was not written by Sviatov's father himself
This is a trap option for those who aren't clear about what the question is asking. Does the argument depend on whether the father wrote the book or not? No. That has nothing to do with it. The father is just used as a reference to help narrow in on when the book was written. What is assumed though is that the family tree reference is accurate, and that's what C does.