The correct answer is option C) Myopia.

Myopia, also known as nearsightedness, is a common vision problem in which distant objects appear blurry while close objects can be seen clearly. It occurs when the eyeball is too long or the cornea (the clear front surface of the eye) is too curved. As a result, light entering the eye is focused in front of the retina instead of directly on it, leading to a blurred image.

Explanation:
To correct myopia, a person needs a diverging lens (concave lens) that helps to spread out the light rays before they reach the eye, allowing them to focus correctly on the retina. In this case, the boy is using spectacles with a focal length of -50 cm. The negative sign indicates that the lens is concave.

The focal length of a lens determines the amount of bending or spreading of light rays. In myopia, the focal length of the concave lens needs to be negative and relatively strong to compensate for the excessive focusing power of the eye. This means that the lens needs to be able to diverge the incoming light rays enough so that they focus properly on the retina.

The focal length of -50 cm suggests that the concave lens used by the boy is relatively strong. This lens helps to spread out the light rays coming from distant objects before entering the eye, allowing them to focus correctly on the retina and providing clear vision.

Summary:
The boy is using spectacles with a focal length of -50 cm, which indicates that he is suffering from myopia or nearsightedness. This condition requires the use of concave lenses to diverge the incoming light rays and correct the focal point, allowing distant objects to be seen clearly.