What was my estimation about Mexico City's population?

The correct answer conveys a specific estimation regarding the population of Mexico City, stating that it was believed to have about 20 million inhabitants. This response gives a clear quantitative figure, which aligns with providing an estimation as asked in the question. It reflects a judgment made based on understanding Mexico City's demographic context.

In contrast, the other choices present different types of estimations that do not directly address the prompt. Some suggest growth or decline, while others give figures that may differ from the specified number. They focus on potential changes rather than a fixed estimate of the current population size, which is why they do not meet the requirement of the question as effectively as the correct answer.