Archaeologists in Egypt have excavated a 5,000-year-old wooden hull that is the earliest surviving example of a “built” boat—in other words, a boat constructed out of planks fitted together—and that thus represents a major advance, in terms of boat-building technology, over the dugout logs and reed vessels of more ancient vintage.
Option elimination -
(A) together—and that thus represents - We either need the second dash to end the description of the "built boat" or the sentence till the end should be a description. We have the 2nd dash here, and that's good as it ends the description and starts the second clause - wooden hull that (goes back to the wooden hull) ......and that (goes back to the wooden hull) thus (means the result of same first characteristics and not a new characteristic)....that and that parallelism to introduce to relative pronoun clauses.
(B) together—and this has represented - "this" is a singular demonstrative pronoun that needs a noun after it. For example, The team has been working tirelessly on the project, and this effort has resulted in a significant increase in productivity. Otherwise, "this" as a singular demonstrative pronoun can refer to "a boat" or "wooden hull." Ambiguous. Moreover, "has represented" is wrong. In line with the simple present construction earlier, we need the simple present construction here - as it still represents.
(C) together, and it represents - we need a dash, and moreover it has an ambiguous reference. "it" refers to "a boat"? Wrong.
(D) together that was representing - use of past continuous is wrong, we are talking about that is still represents.
(E) together to represent - to represent means intent. They were not constructing a boat 5000 years back to show us (5000 years from now) to represent a major technological advance.