Final answer:
The described attachment style is secure attachment, which is the most common and considered the healthiest, where the infant uses the caregiver as a secure base to explore and seeks comfort in distress.
Step-by-step explanation:
The attachment style where the infant is confident to play in an unfamiliar environment as long as the caregiver is present, and is readily comforted by the caregiver during times of distress, is known as secure attachment. This style is characterized by the child using the parent as a secure base from which to explore their surroundings. In the context of the Strange Situation experiment, securely attached children show distress when their caregivers leave but are happy upon their return, seeking comfort from them if they were upset during the separation.