Correct Option: A. January 1966 and December 1983 Detailed Explanation: To understand why option A is the correct answer, we need to look at the history of military coups in Nigeria, particularly focusing on the two significant events that led to the overthrow of civilian governments.

1. January 1966 Coup:
2. The first military coup in Nigeria occurred on January 15, 1966. This coup was led by Major Chukwuma Kaduna Nzeogwu and other military officers. The coup aimed to address widespread corruption and mismanagement in the civilian government led by Prime Minister Sir Abubakar Tafawa Balewa.
3. The coup was initially successful, resulting in the assassination of several key political figures, including Balewa. General Johnson Aguiyi-Ironsi became the head of state following the coup.
4. However, this coup led to significant ethnic tensions, particularly between the northern and southern regions of Nigeria, which would later contribute to further instability.
5. December 1983 Coup:
6. The second significant coup occurred on December 31, 1983, led by Major General Muhammadu Buhari. This coup overthrew the civilian government of President Shehu Shagari, who had been in power since 1979.
7. The Buhari coup was justified by the military as a necessary action to combat corruption and economic mismanagement, similar to the rationale of the 1966 coup. Buhari's regime was marked by strict measures against corruption and a focus on economic recovery.
8. This coup marked the beginning of another prolonged period of military rule in Nigeria, which lasted until the return to civilian rule in 1999.

Why Other Options Are Incorrect:

• Option B: July 1966 and August 1985:
• The July 1966 coup was indeed a significant event, but it was a counter-coup that followed the January 1966 coup. It was led by northern military officers and resulted in the assassination of General Aguiyi-Ironsi. However, this does not represent a coup that toppled a civilian regime; rather, it was a military power struggle.
• The August 1985 coup, led by General Ibrahim Babangida, occurred after the December 1983 coup and was not a direct overthrow of a civilian government.
• Option C: January 1966 and July 1975:
• While the January 1966 coup is correct, the July 1975 coup is not. The July 1975 coup, led by General Murtala Mohammed, overthrew General Yakubu Gowon, who was also a military leader. This does not fit the criteria of toppling a civilian regime.
• Option D: February 1976 and December 1983:
• The February 1976 coup, which resulted in the assassination of General Murtala Mohammed, was also a military coup and did not involve the overthrow of a civilian government. It was a continuation of military rule, and thus does not meet the criteria of the question.

Revision Summary:

• The two military coups that toppled civilian regimes in Nigeria were the January 1966 coup and the December 1983 coup.
• The January 1966 coup led to the overthrow of Prime Minister Tafawa Balewa, while the December 1983 coup removed President Shehu Shagari.
• Other options either refer to military power struggles or coups that did not involve civilian governments.
• Understanding the context and outcomes of these coups is crucial for grasping Nigeria's political history and the impact of military rule on its governance.