Final answer:
Infants can learn through instrumental conditioning by 6 months of age.
Step-by-step explanation:
Infants can learn through instrumental conditioning by 6 months of age, according to the provided information.
At 6 months of age, infants are able to pick up objects, transfer them from hand to hand, and pull themselves into a sitting position. They can also start to show anxiety toward strangers. These behaviors indicate that they have already started learning through instrumental conditioning.