9514 1404 393
Step-by-step explanation:
We have to assume the triangle is a right triangle.
In a 30°-60°-90° right triangle the side lengths have the ratio ...
short side : long side = 1 : √3
This lets you write the proportion ...
x/15 = 1/√3
Multiplying by 15 gives ...
x = 15/√3 . . . . . . matching choice D
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There are other ways to get there. One of them is ...
x = 15·tan(30°) = 15/√3