Reason for high birth rate as the most convincing factor for population growth:

There are several factors that contribute to population growth in the world, but the most convincing factor is a high birth rate. This is due to the following reasons:

1. Natural Population Increase:
- A high birth rate leads to natural population increase, as the number of births exceeds the number of deaths.
- Even with advancements in healthcare and access to medical facilities, there is still a significant proportion of the global population that lacks access to quality healthcare. As a result, the mortality rate remains high, especially in developing countries. However, the birth rate continues to be high, leading to population growth.

2. Cultural and Social Factors:
- Cultural and social factors also play a significant role in high birth rates. In many societies, having a large number of children is considered desirable and a symbol of wealth and status.
- Religious beliefs, societal norms, and traditions often encourage families to have more children, which further contributes to population growth.

3. Lack of Family Planning and Contraception:
- In many regions, there is limited access to family planning services and contraception methods. This lack of availability and knowledge about contraception leads to unintended pregnancies and subsequently, a higher birth rate.
- Family planning education and access to contraception are crucial in empowering individuals to make informed choices about the number and spacing of their children.

4. Economic Factors:
- In some societies, particularly in developing countries, having more children is seen as an economic advantage. Children are often seen as a source of labor and support for the family.
- Additionally, in agrarian economies, a larger population can help cultivate more land and increase agricultural productivity.

Conclusion:
While excellent healthcare (option C) is undoubtedly a significant factor in population growth, it is not as convincing as a high birth rate. The ability to provide quality healthcare does not necessarily lead to a decrease in birth rates unless accompanied by effective family planning measures and cultural shifts. High birth rates have a direct impact on population growth and can outweigh other factors in determining population increase.