I will go with option E.Meaning: An influx of international aid to a war-torn region was not enough to provide food for many indigenous people. As a result, many[indigenous people] left home to travel west where other tribal groups
historically had experienced fewer difficulties.
A big clue to solving this question is the word historically. Historically suggests an action or an account that already took place and is completed in the past. So it requires the past perfect tense. Only options D and E use the past perfect tense. Based on this we can eliminate A, B, and C.
Between D and E, let's look at the context of option D. Option D suggests that many[indigenous people] left home to travel west
in search of tribal groups who had historically experienced fewer difficulties. It implies that the migrants left home to search for other tribal groups in the west. This changes the meaning of the sentence. It is not the other tribes that the migrants were searching for but rather the migrants were in search of a place where other tribal groups had experienced fewer difficulties. Based on this change in meaning, option D can be eliminated and we are left with option E as the answer.
Despite the influx of international aid to a particularly war-torn region of East Africa during the waning months of 2006, in early 2007, many indigenous people unable to find food left their home to travel west where other tribal groups historically [
had] experienced
less difficulties producing food, water, and land.
(A) their home to travel west where other tribal groups historically [
had] experienced
less difficulties
(B) home to travel west where other tribal groups historically [
had] experienced fewer difficulties
(C) their home to travel west where other tribal groups historically [
had] experienced fewer difficulties
(D) home to travel west
in search of other tribal groups who historically
had experienced fewer difficulties
(E) home to travel west where other tribal groups historically
had experienced fewer difficulties