The most accurate response to why the Belgian Congo was unable to attain peace immediately after gaining independence is: The Belgian military leaders maintained power over Congolese soldiers.
Following independence in 1960, the Congo faced significant political instability, partly due to the continued influence and control of Belgian military leaders, who remained involved in the Congolese military and governance structures. This led to power struggles, a lack of effective leadership, and prolonged conflict.
While Mobutu's leadership also contributed to ongoing challenges later on, the immediate issue of Belgian military influence was more directly linked to the inability to achieve peace right after independence.