D is the answer.Premise: Explorers of the Northern regions in the early 1700s observed the natives playing an instrument similar to the mandolin. The instrument was strung with horse hair. Horses were not introduced into the New World until the 1500s.
Conclusion: Thus, we can conclude that natives developed the instrument sometime between the introduction of horses to the New World and the time of the explorers in the early 1700s.
The author is concluding that the instrument was developed by the natives sometime between the introduction of horses to the New World and the tie of the explorers in the early 1700s. This conclusion is based on the premise that horses were not introduced into the New World until the 1500s, hence the instrument ought to be invented after the introduction of horses. What can flaw this argument is that the natives developed the instrument earlier on than the period suggested by the authors conclusion, except that instead of using horse hairs, the natives used a different material as strings for the instrument. So, for the argument to hold water, then it has to be assumed that only horse hairs were used as strings for the instrument since its invention.
This is exactly what option D states, hence the answer has to be D in my view.
(A) Natives used the mandolin-like instrument in all their religious events. Irrelevant.
(B) Using horse hair in the mandolin-like instrument was one of the natives’ earliest uses of horse hair. This doesn't save the argument from a potential flaw whereby the instrument was developed earlier before horses were introduced using a different material as string prior to time that the explorers visited the region. Hence B is not the necessary assumption on which the argument relies.
(C) This instrument was used by natives throughout North America. Irrelevant.
(D) Since it was first developed, the instrument was made with horse hair. Correct.
(E) Explorers in the 1700s were the first to document natives’ use of horse hair. Irrelevant.