Who firmly believed that a happy prosperous national existence could be…

Concept

Belief in a "capitalist social system" as the basis of national prosperity means trusting private ownership of the means of production and a market-driven, profit-oriented order to generate well-being. To answer, test each named leader against this criterion: did that person advocate capitalism (private enterprise, market order) as the foundation of national life, or did they prefer an alternative order such as socialism, a planned/mixed economy, or decentralised self-reliance?

Applying the test to each leader

• Mahatma Gandhi: advocated a decentralised economy of self-reliant villages, small-scale industry and trusteeship, and was a sharp critic of large-scale industrial capitalism. He did not ground national prosperity in a capitalist system.

• Jawaharlal Nehru: championed socialism, central planning and a mixed economy with state control of key industries. His vision rejected pure capitalism as the basis of national life.

• Subhas Chandra Bose: envisioned a socialist state with state control over the means of production; he too did not treat capitalism as the foundation of national prosperity.

Cross-check and result

All three named leaders leaned toward socialist, planned, mixed or self-reliance models rather than treating capitalism as the firm basis of national prosperity — Nehru's mixed economy did retain some space for private enterprise, and Gandhi's trusteeship did not reject private property outright, but in no case is a firm belief that prosperity rests on a capitalist social system (as the question specifies) documented. Since none of the three firmly held that specific belief, the option that selects no individual leader follows from this elimination.