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

The most efficient factors in Mexico's climate is the ocean current - for example the Californian current brings cold water to Mexico and lowers the temperatures. The correct answer is d).

Elevation also plays a role. However, population density does not influence the climate.

An informal structure in a bureaucracy would be something like so-called "office politics". where there's a formal structure set up for the organization, but then an informal subculture as well that has a way of organizing the bureaucracy further. this can be positive because it sets up an added degree of organization, albeit informal, within the bureaucracy.