In the 19th century, increasing food production in the world required more than just clearing land. Several factors played a crucial role in enhancing agricultural productivity and meeting the growing demands for food. These factors include:

a) Railways to link agricultural regions, harbors to be expanded or built for new cargoes:
- Railways played a significant role in improving transportation and connectivity between different agricultural regions. They facilitated the movement of agricultural produce, machinery, and manpower, thus reducing transportation costs and time.
- The expansion or construction of harbors was necessary to accommodate larger ships and increase the efficiency of trade. This allowed for the import and export of agricultural goods to and from different parts of the world, further boosting food production.

b) Building homes and settlements for those working on land:
- Providing suitable housing and settlements for agricultural workers was crucial to ensure a stable workforce and sustained agricultural production. Adequate housing facilities helped attract and retain workers, leading to increased efficiency and productivity in farming activities.

c) Capital and labor:
- Capital investment was necessary to improve farming techniques, infrastructure, and machinery. This included investments in irrigation systems, improved tools and equipment, and the introduction of new farming methods. Capital also enabled the purchase of seeds, fertilizers, and other inputs required for enhanced production.
- Adequate labor supply was essential for agricultural expansion. Increasing population and urbanization during the 19th century led to a shortage of agricultural labor. Efforts were made to encourage migration of people from rural to agricultural areas to meet the growing demand for labor.

d) All the above:
- The correct answer is option 'D' because all the mentioned factors were interdependent and played a significant role in increasing food production in the world. Railways and expanded harbors facilitated transportation and trade, while housing and settlements provided a stable workforce. Capital investment and labor supply were essential for implementing advanced farming techniques and expanding agricultural operations.

In conclusion, increasing food production in the 19th century required a comprehensive approach that encompassed clearing land, establishing transportation networks, providing suitable housing for workers, and ensuring sufficient capital and labor resources. These factors worked together to enhance agricultural productivity and meet the growing demands for food.