Which of the following is not a core industry ?

The correct answer is (d), leather.

Core industries are industries that are essential to the functioning of an economy. They are typically industries that produce basic goods and services that are used by other industries.

Fertilizers are essential for agriculture. Steel is used in construction and manufacturing. Electricity is used to power homes and businesses. Leather is used to make shoes, bags, and other products.

Of the above options, only leather is not essential to the functioning of an economy. It is a secondary industry that produces goods that are not essential for basic needs.