Step 1: Understand the context of US involvement in Libya.
The major US and NATO involvement in Libya occurred in 2011, during the Libyan Civil War, which aimed to overthrow Muammar Gaddafi's regime. This intervention was officially justified under the UN Security Council Resolution 1973, which authorized a no-fly zone and "all necessary measures" to protect civilians.
Step 2: Evaluate the given options in light of historical and geopolitical analysis.
(1) Supporting the Gaddafi rule: This is incorrect. The US and NATO actively sought to remove Gaddafi from power, not support him.
(2) Suppression of rebels: This is incorrect. The intervention was primarily aimed at protecting civilians from Gaddafi's forces and, in effect, assisting the rebels, not suppressing them.
(3) Desire to control the large oil reserves: Libya possesses Africa's largest proven oil reserves. Geopolitical analysts and critics often argue that control or influence over these strategic resources, coupled with the desire to maintain regional stability favorable to Western interests, played a significant underlying role in the intervention, beyond humanitarian concerns. While not the publicly stated reason, it is widely considered a major strategic interest.
(4) Stopping United Nations Organisation's intervention: This is incorrect. The US intervention (along with NATO) was conducted under a UN Security Council resolution, meaning it was a UN-authorized intervention, not an action to stop UN intervention.
Step 3: Conclude the most plausible main reason.
While humanitarian concerns were the stated reason for intervention, the strategic importance of Libya's vast oil reserves is widely considered by geopolitical analysts to be a primary underlying reason for the significant interest and involvement of the United States and other Western powers in Libya's internal affairs.