Final answer:
C. 6 months. An infant is typically able to sit up on their own at around six months of age.
Step-by-step explanation:
The milestone of an infant being able to sit up on their own typically occurs at around six months of age. At this stage, the infant can pull themselves into a sitting position and has improved vision, allowing them to notice colors and discriminate depth. It is important to note that developmental milestones can vary among infants, with most babies achieving this milestone between 5 and 9 months old.