What is the approximate difference in elevation from point C to point D?

The difference in elevation between the two points is around 400 meters.

Explanation:

We can see that point C is located very close to the 1500 meter mark, while point D is located between the 1,000 and 1,200 mark, more specifically, very close to the 1,100 mark (even if this mark is not labeled).

We can assume then, that point D elevation is very close to 1,100 meters, and from a simple substraction: 1,500 - 1,100 we obtain the 400 figure that is the answer to this question.