In a debate about nationalising mines, a policymaker argues that mineral wealth should be collectively owned to prevent exploitation by private elites. Which school of thought most closely supports this argument?

The policymaker's argument for mineral wealth to be "collectively owned to prevent exploitation by private elites" aligns with a specific economic ideology.

• Keynesian economics focuses on government intervention to manage aggregate demand and stabilize the economy, not necessarily collective ownership of resources.
• Classical economics emphasizes free markets, private property, and minimal government intervention.
• Monetarist economics focuses on controlling the money supply to achieve economic stability, also generally favoring free markets.
• Marxist theory, however, critiques capitalism for its inherent exploitation of labor by the capitalist class (private elites) and advocates for the collective ownership of the means of production (like mines) to eliminate such exploitation and achieve a classless society.

Therefore, the school of thought that most closely supports this argument is Marxist.

The final answer is $\text{D. Marxist}$.