Adjustment of the lens to focus on objects close to the viewer is called a. convergence. c. focusing. b. visual accommodation. d. constriction.

First, let's take a look at each option and see if it corresponds to the description given in the exercise: a. Convergence: It refers to the movement of the eyes towards each other when focusing on a close object. This is not the correct answer, as it does not relate to the lens adjustment. b. Visual accommodation: It is the process by which the eyes' lenses change their shape to focus on objects at different distance. This seems to be the correct answer. c. Focusing: It is a more generic term referring to the process of making an image clear, which involves the lens, but it is not specific to eye lens adjustment. d. Constriction: It is the narrowing of an opening or passage; this term is generally related to the pupil size and not the adjustment of the lens for focusing on close objects.

Based on the analysis of each option, the correct answer to the question is: b. Visual accommodation: This term best describes the adjustment of the lens to focus on objects close to the viewer.