The correct answer is: B. have become
The US government allowed 7000 Navajo Indians to return to their homeland in 1868. Since then, they have become the largest and richest Indian tribe.
The other options are incorrect because they do not accurately reflect the timeline of events. Option A, “allowed,” is in the past tense, but the Navajo Indians did not become the largest and richest Indian tribe until after they returned to their homeland in 1868. Option C, “has allowed,” is in the present perfect tense, but the Navajo Indians did not become the largest and richest Indian tribe until after they returned to their homeland in 1868. Option D, “are becoming,” is in the present progressive tense, but the Navajo Indians did not become the largest and richest Indian tribe until after they returned to their homeland in 1868. Option E, “become,” is in the simple present tense, but the Navajo Indians did not become the largest and richest Indian tribe until after they returned to their homeland in 1868.