When does upward growth of cities typically occur?

a.

when water resources are limited

b.

when waste production is a problem

c.

when available land is limited

d.

when population exceeds 100,000

C. When available land is limited 

When the land is limited, people stop building outwards, they build upwards and make tall skyscrapers and tall buildings because there isn't any other place they can expand to.

Hope this helps :)

Iwould say the answer is d