Heya mate!
If you observe the figure, the longest side always resides the opposite place of the right angle. Accordingly the perpendicular is 5√3 units in measure.
With this, the hypotenuse is x and the base is y. We need to find y. By doing some smart work! Let's guess the trigonometric ratio that we give us the relation between y and 5√3 because one of the angle given is 60°
⇛ tan a = Perpendicular / Base
Plugging the values of a, and Perpendicular to find the base of the triangle i.e. y.
⇛ tan 60° = 5√3 / y
⇛ √3 = 5√3 / y
⇛ y = 5√3 / √3
⇛ y = 5 units
Hence, the Correct Option is Option A.