Okay, here we have this:
Considering the provided figure, that we know an angle, the leg opposite a is an angle and we want to know the hypotenuse, we can use the trigonometric ratios since it is a right triangle, so we obtain the following:
The trigonometric ratio that relates an angle, its opposite leg and the hypotenuse is the sine, therefore that is the one we will use, so we have:
Finally we obtain that the correct answer is the first option (A).