The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This is due to
A. presbyopia
B. accommodation
C. near-sightedness
D. far-sightedness

Hint The question is asking about the ability of the human eye to focus on objects at different distances; a phenomenon associated with the normal functioning of the eye. Therefore, we can eliminate the options that are related to optical flaws in vision.

Complete step-by-step answer
Accommodation is the process by which the eye changes optical power to maintain a clear image or focus on an object as its distance varies. In other words, the process by which the human eye can focus objects at different distances by adjusting the focal length of the eye lens is called accommodation. This change in focal length is achieved by flattening or thickening of the eye lens.
The other three options mentioned are just different conditions in which the human eye is unable to focus on an object.

Additional Information
The young human eye can change focus from distance (infinity) to as near as 6.5 cm from the eye.
Nearsightedness or Myopia is a condition of the eye in which parallel rays are focused in front of the retina, objects being seen distinctly only when near to the eye.
Far-sightedness, also known as long-sightedness, hypermetropia, or hyperopia, is a condition of the eye in which light is focused behind, instead of on, the retina with accommodation being at rest.
Presbyopia is long-sightedness caused by loss of elasticity of the lens of the eye, occurring typically in middle and old age.

Note ‘To accommodate’ literally means to adjust or adapt to someone or something, the eye also adjusts or accommodates its lens to see objects at different distances clearly.