What Was the Research Question Harry Harlow Addressed in His

What was the research question Harry Harlow addressed in his classic study of attachment?

A) Are infants capable of forming multiple attachments?
B) Are infants attached to mothers because mothers are entertaining?
C) Are infants attached to their mothers because mothers serve as their primary source of food?
D) Are infants biologically predisposed to form attachment bonds with their mothers, regardless of how the mothers behave?