Mawsynram is the wettest place on Earth with an annual average rainfall of around 11,871 millimeters (467.4 inches), making it the perfect answer for this question. Located in the state of Meghalaya in northeastern India, Mawsynram experiences an extremely high amount of rainfall due to its geographical location and topography.

Geographical Location:
Mawsynram is situated in the Khasi Hills, one of the wettest regions in the world. Its proximity to the Bay of Bengal plays a significant role in the heavy rainfall it receives. The moisture-laden winds coming from the sea get trapped by the hills, leading to a high amount of precipitation.

Topography:
The topography of Mawsynram also contributes to its high rainfall. The region is characterized by deep valleys and steep slopes, which act as natural channels for the rainwater to flow down. This terrain allows for quick runoff, resulting in a higher volume of rainfall.

Monsoon Influence:
Mawsynram is greatly influenced by the southwest monsoon, which occurs from June to September. During this period, the region experiences heavy rains, often resulting in flooding. The monsoon winds bring abundant moisture from the Bay of Bengal, leading to the continuous rainfall in Mawsynram.

Comparison with Other Cities:
While Mumbai (option A), Chennai (option B), and Thiruvananthapuram (option D) do receive significant rainfall, they are not as wet as Mawsynram. Mumbai experiences a tropical wet and dry climate, with an average annual rainfall of approximately 2,200 millimeters (86.6 inches). Chennai has a tropical wet and dry climate as well, with an average annual rainfall of around 1,400 millimeters (55.1 inches). Thiruvananthapuram, located in the state of Kerala, has a tropical monsoon climate with an average annual rainfall of about 1,800 millimeters (70.9 inches). Therefore, none of these cities receive as much rainfall as Mawsynram.

In conclusion, Mawsynram is the wettest place on Earth due to its geographical location, topography, and the influence of the southwest monsoon. Its annual average rainfall of 11,871 millimeters makes it the perfect answer for this question.