Final answer:
An infant should be able to fix on and follow a target by three months of age, when their color perception and ability to follow objects with their eyes have developed substantially.
Step-by-step explanation:
The nurse is testing an infant's visual acuity. By which age should the infant be able to fix on and follow a target? The correct answer is by three months of age. During the initial months after birth, an infant's vision is rapidly developing. While newborns have vision that is still immature and their depth perception is limited, they are mostly able to focus on objects within 46 cm (18 in.), which is the perfect distance to see a mother's face when feeding. By around two months, infants develop the ability to hold their head erect and steady, and they start showing signs of social engagement, like smiling at their parents. As they reach four months, infants are able to roll from front to side, lift their head 90 degrees, and follow objects with their eyes.
It's at around three months when an infant's color perception improves significantly and they can distinguish several colors. By four months, their vision has developed enough to not just follow objects but also turn their head toward sounds. Therefore, by three to four months, infants typically have the visual acuity to fixate on and track a target.