Step 1: Recall the meaning of public sector.
A public sector enterprise is one that the government owns and runs, mainly to serve the public rather than to chase profit.
Step 2: Check each company's ownership.
Birla Cement, Tata Iron and Steel Company, and Reliance Industries are all run by private business families or corporate groups, so they belong to the private sector.
Step 3: Check Indian Railways.
Indian Railways is owned and run by the Government of India through the Ministry of Railways, providing transport as a public service to the whole country.
Step 4: State the final answer.
Hence, the example of the public sector here is option (D): Indian Railways.