Harry Harlow discovered from his studies on attachment that isolated infant monkeys prefer?

Harry Harlow's studies on attachment with infant monkeys revealed that isolated infant monkeys preferred the cloth surrogate mother over the wire surrogate mother that provided food.

This was a groundbreaking discovery because it challenged the prevailing belief that attachment was solely based on the satisfaction of basic needs like food. Harlow's research demonstrated the importance of contact comfort and emotional security in the development of healthy attachment relationships.