The question asks about the primary factor contributing to the growth of the tractor industry in India in recent years. There are several potential factors listed, and each needs to be analyzed to determine which one has had the most significant impact.

1. Mechanization of agriculture has increased:
   • This option states that the level of mechanization in Indian agriculture has increased. Mechanization involves the use of machinery, like tractors, to perform agricultural tasks more efficiently than manual labor. As farms grow larger and the demand for efficiency increases, more farmers are likely to invest in tractors to enhance productivity.
   • The government's push for modernizing agriculture also promotes the use of machinery, aiming for better yield and cost-effectiveness.
   • This is the most convincing factor leading to the growth of the tractor industry.
2. Urbanization has remained constant:
   • While urbanization can affect agricultural practices due to reduced rural workforce availability, the option states it has remained constant, implying no significant change driving an increase in tractor sales.
   • Therefore, it does not directly contribute to increased tractor demand.
3. Manual labor availability has declined:
   • A reduction in manual labor availability could push farmers to invest in machinery like tractors. However, without explicit reference to its impact on tractor growth in this context, it is less persuasive than mechanization, which directly promotes the use of tractors.
4. Export demand for tractors has decreased:
   • A decrease in export demand implies reduced international sales, possibly leading to stockpile issues, harming domestic growth. Thus, this factor does not contribute positively to growth within India.

Considering all options, the increase in mechanization of agriculture in India appears to be the most significant factor contributing to the recent growth of the tractor industry. Mechanization directly supports increased demand for tractors as farmers strive to enhance efficiency and productivity.

Thus, the correct answer is: Mechanization of agriculture has increased.