Answer:

In India, several industries are reserved for the public sector, which means that these industries are owned and operated by the government. However, there are certain industries that are not reserved for the public sector, and one of them is defense. Let's discuss each option in detail:

a) Atomic energy:
Atomic energy is an industry that is reserved for the public sector in India. The Atomic Energy Act, 1962, gives the government the exclusive right to develop, control, and use atomic energy. The Department of Atomic Energy (DAE) is responsible for the development and regulation of atomic energy in the country. All atomic energy-related activities, including the establishment of nuclear power plants, research and development, and mining of atomic minerals, are managed by the public sector under the control of the DAE.

b) Railways:
Railways is another industry that is reserved for the public sector in India. Indian Railways is a government-owned railway system that operates as a public sector undertaking. It is the fourth-largest railway network in the world and is responsible for the operation and maintenance of rail transport in the country. Indian Railways plays a crucial role in the transportation of goods and passengers across the country and is managed by the Ministry of Railways.

c) Defense:
Defense is the correct answer as it is not reserved for the public sector in India. Defense is an industry that involves national security and the protection of the country against external threats. In India, defense-related activities are primarily carried out by the Ministry of Defense, which includes the Indian Armed Forces (Army, Navy, and Air Force) and various defense research and development organizations. While the government plays a significant role in defense procurement and policy-making, there is also participation from the private sector in defense manufacturing and technology development.

d) Substances specified in the schedule to the notification of the government of India in the department of atomic energy:
This option is not relevant in the context of industries reserved for the public sector. It refers to substances specified in the schedule to the notification of the government of India in the department of atomic energy, which is related to atomic energy and its regulation. However, it does not specifically indicate an industry that is reserved for the public sector.

In conclusion, defense is the industry that is not reserved for the public sector in India, while atomic energy and railways are industries that are reserved for the public sector.