Answer:
The caregiver leaves the infant with a stranger and returns later
Explanation:
in order to measure the attachment, the infants need to be put in the situation where the person that they trust as their primary caretaker suddenly leave them and make them vulnerable to dangers.
In order to do this, the main caregivers need to leave the infants with a stranger and return later. The researchers then can measure the heart rate and find out how fast the infants will start to cry as soon as the were left by the caretakers.