Concept: The military coup of 1966 was prompted by the political instability from electoral crises and breakdown of law and order.
Explanation:The 1964 General Elections were marred by widespread rigging and political tension.
The disputed results created a serious crisis that threatened the unity of Nigeria.
This crisis deepened with the violent Western Regional Election of 1965.
The resulting disorder made the political situation unbearable.
As a result, the military intervened in January 1966.
Therefore, the key reason was the unresolved crisis from the 1964 General Elections.
Answer: D. crisis of the 1964 General Elections.