Based on the information provided:
Question 1: The area that lost the most in percentage decline is likely "Valley of Mexico" (assuming this corresponds with the highest percentage shown in the chart, which I cannot see).
Question 2: To determine how many years it took for this decline in population, please select the number that corresponds to your data; it could be 6, 9, 7, or 5.
Question 3: The most plausible cause for a massive decline in population could be "Disease," as this historically has caused significant population decreases in many regions. However, relocation or not enough babies born could also be contributing factors depending on the context.
Please verify the answers with the chart data you have for confirmation.