Final answer:
A healthy infant typically starts smiling in response to smiling and talking at around 2-3 months of age. By six weeks, they begin to smile and make vocal sounds, and the development continues with sitting, crawling, and eventually standing and walking by the end of the first year.
Step-by-step explanation:
At what age can a healthy infant smile in response to smiling and talking? The correct answer is a. 2-3 months. Healthy infants typically begin to smile at their parents and other familiar people during the first two months after birth. By around 6 weeks after birth, infants start smiling and making vocal sounds. As development progresses, by six months of age, infants have learned to sit and may be starting to crawl, and they also begin babbling sounds like "mama" or "dada." By ten months, they advance to wiggling and crawling, and by the end of the first year, they may be standing with support and even starting to walk.