The correct option is B. Chad. Detailed Explanation:

1. Historical Context: In the late 1970s and early 1980s, Chad was embroiled in a civil war that involved various factions vying for control of the country. The conflict was exacerbated by the involvement of foreign powers, including Libya, which had sent troops to support one of the factions, the Goukouni Oueddei government.
2. Role of the Organization of African Unity (OAU): The Organization of African Unity, established in 1963, aimed to promote unity and cooperation among African states. One of its key functions was to mediate conflicts and promote peace on the continent. In response to the instability in Chad and the presence of Libyan forces, the OAU decided to intervene.
3. Nigeria's Involvement: In 1981, Nigeria, as a member of the OAU, contributed troops to a peacekeeping force that was deployed to Chad. This force was tasked with replacing Libyan troops and stabilizing the situation in the country. Nigeria's involvement was part of a broader effort by African nations to address conflicts within the continent without relying on external powers.
4. Outcome: The OAU peacekeeping mission in Chad was significant as it represented a collective African response to a crisis in one of its member states. The mission aimed to restore order and support the transitional government in Chad, which was struggling against various rebel groups.

Why the Other Options are Incorrect:

• A. Somalia: While Somalia has faced significant conflict, particularly during the civil war in the early 1990s, Nigeria's involvement in peacekeeping efforts there occurred later and was not related to the OAU's actions in 1981. The OAU was not directly involved in peacekeeping in Somalia at that time.
• C. Ethiopia: Ethiopia has had its share of conflicts, particularly during the Ethiopian Civil War (1974-1991). However, the OAU's peacekeeping efforts in Ethiopia were not prominent in 1981, and Nigeria did not send troops there in the context of replacing Libyan forces.
• D. Zaire: Zaire (now the Democratic Republic of the Congo) has also experienced significant political turmoil, but the OAU's peacekeeping efforts in Zaire were not a focus in 1981. The situation in Zaire was different, and Nigeria's military involvement was not related to replacing Libyan forces there.

Summary of Key Points:

• Nigeria participated in the OAU peacekeeping force in 1981 to replace Libyan troops in Chad.
• The OAU aimed to stabilize Chad amidst civil conflict and foreign intervention.
• Nigeria's involvement was part of a broader African initiative to address internal conflicts.
• Other options (Somalia, Ethiopia, Zaire) do not relate to Nigeria's actions in 1981 regarding Libyan forces.

This understanding of Nigeria's role in Chad during the OAU peacekeeping mission is crucial for grasping the dynamics of African politics and international relations during that period.