The table below shows the population of the three countries in 2011.

To find the combined population of the United States and Mexico, you simply need to add their populations together.

Population of United States = 3.1 * 10^8
Population of Mexico = 1.2 * 10^8

Combined population = 3.1 * 10^8 + 1.2 * 10^8 = 4.3 * 10^8

Therefore, the combined population of the United States and Mexico in 2011 was 4.3 * 10^8, which is option D.