The right answer is the mandate to marry within one's tribe. Even though Nnaemeka knew that his decision to marry a woman from another tribe would upset his father, he thought that his annoyance would eventually pass away. But he had not realized that his was a decision without precedent, and this caused surprise, indignation, and sadness among his people, who, stuck to tradition, saw in Nnaemeka's act a reflection of their Lord's cautionary teachings ("Sons shall rise against their fathers") and a bad omen ("It is the beginning of the end"), hence illustrating their conviction in the mandate to marry within one's tribe.
Answer: The right answer is the mandate to marry within one's tribe. Even though Nnaemeka knew that his decision to marry a woman from another tribe would upset his father, he thought that his annoyance would eventually pass away.