Today, the world’s number one problem is:  A. Pollution  B. Population explosion  C. Nuclear proliferation  D. Natural calamities

This question seems to be asking for an opinion rather than a factual answer, as the "number one problem" in the world can vary greatly depending on one's perspective, values, and the specific criteria used to determine what constitutes a "problem".

However, I can provide a brief analysis of each option:

A. Pollution: This is a significant global issue that affects air, water, and land quality, contributes to climate change, and impacts both human health and biodiversity.

B. Population explosion: Rapid population growth can lead to resource depletion, environmental degradation, and challenges in providing adequate services such as education and healthcare. However, it's worth noting that population growth rates are slowing globally and this issue is less prominent than it once was.

C. Nuclear proliferation: The spread of nuclear weapons is a major concern because of the potential for catastrophic damage. However, international treaties and diplomatic efforts aim to control this.

D. Natural calamities: These are certainly problematic, causing loss of life and property, but they are not something humans have much control over, unlike the other issues listed.

In conclusion, all of these are significant problems facing the world today, and different people might rank them differently based on their own perspectives and concerns.