Answer:
Sine; 

Explanation:
The figure is included below as attachment. A right-angled triangle is formed by considering the variables mentioned in the statement of the problem. The trigonometric function that uses all the mentioned variable is the sine, which is:

Where:
 - Angle of elevation of the sun.
 - Angle of elevation of the sun.
 - Height of the kite off the ground.
 - Height of the kite off the ground.
 - Length of the kite string.
 - Length of the kite string.
The height of the kite can determined by clearing the corresponding variable in the previous formula:
