By infants have become attached to familiar people who have responded to their needs.

By the second half of first year , infants have become attached to familiar people who have responded to their needs.

Explanation:

Children are considered attached when they tend to seek proximity and contact with a specific caregiver in times of distress, illness and tiredness. Attachment to a protective caregiver helps babies regulate their negative emotions in times of stress and distress and explore the environment even when it contains frightening stimuli. Attachment, which is an important developmental milestone in a child's life, remains important throughout life.

Babies begin to cling to family members who have responded to their needs from the second half of the first year of life.

because the early people found out about it.