Infants about 6 to 9 months in age experience crying and fussing when their parents leave and they are left with the baby-sitter. this phenomenon is known as

I believe the answer is: separation anxiety.

separation anxiety refers to the negative emotion that an individual felt when they're separated from the people to whom they have a strong emotional connection with. This is very common among infants between 6 - 18 months year old when they're separated from their primary caretakers.