The correct answer is (d) Social services (excluding rural housing).
The Union Budget 2011-12 allocated Rs. 1,96,220 crore to social services, which is 24.2% of the total budget. This includes education, health, family welfare, nutrition, social security, and welfare of SCs/STs.
Rural development was allocated Rs. 1,40,000 crore, which is 18.2% of the total budget. This includes agriculture, rural development, Panchayati Raj, and drinking water and sanitation.
Energy was allocated Rs. 1,20,000 crore, which is 15.7% of the total budget. This includes power, coal, petroleum and natural gas, and renewable energy.
Transport was allocated Rs. 1,00,000 crore, which is 12.9% of the total budget. This includes railways, highways, shipping, and aviation.
The other options are incorrect because they do not represent the largest outlay in the Union Budget 2011-12.