Final answer:
The correct answer is option 4) 6 months.
Step-by-step explanation:
The voluntary grasp of an infant typically develops around six months of age. At this stage, an infant is not only able to pick up objects but also transfer them from hand to hand. Moreover, the infant's cognitive abilities have developed to the point where they can sit unassisted and begin to show understanding of object permanence.
It is a time of significant growth both physically, with the baby's teeth beginning to come in, and cognitively, as they start engaging more with their environment, responding to people, making vocal sounds such as babbling, and eventually, saying a few words like "mama" and "dada".