This is a strengthen question. The argument claims that, because of the evidence in a burial site, both the conquering Egyptians and the conquered Nubians shared in the ruling of Nubia. The argument assumes that all of the evidence from the burial site stems from an era after Egypt conquered Nubia. If some of the evidence predated that time, the fact that some people were buried according to Nubian tradition would not say anything about who ruled the region after Egypt took over. Consider each choice, looking for one that strengthens that assumption:

(A) This choice doesn't have to do with the burial of Egyptians or Nubians, so it is outside of the scope.
(B) This choice is also outside of scope, as it has to do with burials in Egypt, where the local culture and the ruling culture were (as far as we know from the passage) the same.
(C) This is correct. This confirms the assumption that the evidence from the burial site pertains to the period after Egypt took over, solidifying the claim that Nubians had some control over burial practices even after the area was conquered.
(D) The number of people buried at the site is unimportant; the focus is on the burial practices used.
(E) Given the reference in the passage to "high-ranking Egyptians," it seems likely that the argument refers mainly to colonial officials. Thus, the claim that few others moved to Nubia is not relevant to the argument.