The correct answer is option 'C' - The Green Revolution.

Explanation:
The expansion of the fertilizer industry can be attributed to several factors, but the most significant one is the Green Revolution. Let's discuss each option in detail:

a) Liberalization and foreign direct investment:
While liberalization and foreign direct investment can contribute to the growth of various industries, including fertilizers, they are not the primary factors that led to the expansion of the fertilizer industry. Liberalization policies and foreign investments can create a conducive environment for industrial growth, but they do not directly contribute to the specific expansion of the fertilizer industry.

b) MNCs (multinational corporations):
Similarly, multinational corporations can play a role in the growth of the fertilizer industry by bringing in advanced technologies, expertise, and investment. However, their involvement alone does not explain the expansion of the industry. MNCs may have contributed to the growth but cannot be considered as the sole reason.

c) The Green Revolution:
The Green Revolution, on the other hand, played a pivotal role in the expansion of the fertilizer industry. The Green Revolution refers to a series of technological advancements and agricultural practices that took place in the mid-20th century, primarily in developing countries. It aimed to increase agricultural productivity through the adoption of high-yielding crop varieties, modern irrigation methods, and the use of fertilizers and pesticides.

The Green Revolution brought about a significant increase in agricultural production, particularly in countries like India, Mexico, and the Philippines. To support the high-yielding crop varieties, farmers began using chemical fertilizers on a large scale. This led to an increased demand for fertilizers, which in turn stimulated the growth of the fertilizer industry.

The increased use of fertilizers during the Green Revolution helped farmers achieve higher crop yields and overcome food shortages. It transformed agriculture and contributed to economic growth in many developing countries.

In conclusion, while factors like liberalization, foreign direct investment, and multinational corporations can contribute to the growth of the fertilizer industry, the primary reason for its expansion is the Green Revolution. The adoption of high-yielding crop varieties and the increased use of fertilizers during this period led to a surge in fertilizer demand, thereby driving the growth of the industry.

All of the above.