Hint: The retina is the part of the eye where light focuses and images are formed (similar to film of a camera). All light rays coming from infinity striking a lens focus at the focal point of the lens.
Complete Step-by-Step Solution: Generally, when the light rays coming from infinity strikes a lens, they focus at (or appear to originate from) the focal point of the lens. For convex lenses, they converge at focal point, for concave lenses, they appear to diverge from focal point. The eye lens is similar to a convex lens. The distance between the focal point of a lens and the lens is called the focal length. Now, for humans to see, light coming from objects must focus on the retina as this is the light sensitive part of the eye. This in other words implies that the lens of the eye must be placed such that the focal point coincides exactly with the retina of the eye. Thus, the maximum focal length of the eyes must be equal to the distance from the retina, in our case \[x\].
Hence, The correct option is A
Note: We should note that only light from infinity, (i.e. very far from the eye relative to the focal length) focuses at the focal point. Hence, dependent on our eye lens alone, near objects would be blurry. The eye adapts to this by having a thin layer of membrane called the cornea which refracts light from near objects such that they become parallel to the lens of the eye. However, the cornea has its limit and extremely near objects (like when you place a book very close to your eye) cannot be clearly seen.