The ability of the eye to adjust its focal length to see objects clearly at different distances is called accommodation. This is achieved by changing the shape of the lens. While "contrasting illuminations" involves other mechanisms like pupil size adjustment (controlled by the iris) and adaptation of photoreceptor cells, "accommodation of eye" is the term that best describes the eye's overall ability to adjust to different viewing conditions presented in the question, especially the "different distances" aspect, and is the only adaptive mechanism listed among the options.