Is the key factor in mexico’s climate.
a. coastline
c. population density
b. elevation
d. ocean current

The correct answer is : B. elevation

Usually, for each 1,000 rise in elevation, there would be A 4 Fahrenheit drop in temperature.

Even though the Mexico's elevation differ according to the Area, a huge part of Mexico has a low elevation, which create a very hot climate

Elevation is the key factor in Mexico's climate. The correct option among all the options that are given in the question is option "b". The climate of Mexico is mainly affected by elevation as well as its proximity to the equator. There are other things that effect like population, industrial density  etc. Large bodies of water also effect the climate.