The correct word to complete the sentence is "amicable."
The sentence would read: The Curies' amicable collaboration helped to unlock the secrets of the atom.
Explanation:
The word "amicable" means friendly, harmonious, or characterized by goodwill. In this context, it describes the nature of the relationship between Marie and Pierre Curie as they worked together in their scientific endeavors. Their collaboration was not only productive but also marked by mutual respect and a positive working relationship, which was essential in their groundbreaking discoveries about radioactivity and the atom.
