let's analyze each option and see the role which the bolded phrase plays in this argument.

Option A - Incorrect
This is not true. The bolded phrase is not the premise of the argument, instead, the premise is "The spread of Avian flu from animal to humans has been well-documented, but less understood is the mechanism by which it is spread from one bird species to another".

Option B - Incorrect
The bolded phrase is straightforward on what the author concluded. It doesn't contradict the main point of the passage, rather, it expresses it further for a solution.

Option C - Incorrect
This is similar to option B in explanation

Option D - Correct
This bolded phrase "in order to avoid a worldwide epidemic of Avian flu, scientists must make that study a first priority" is the conclusion of this argument as it provides a final resolution to avoid a worldwide epidemic of Avian flu.

Option E - Incorrect
The bolded phrase is not an assumption that the author made but rather the conclusion