Answer:
partner
Step-by-step explanation:
"Collaborator" = person who works with another.
There are some synonyms for the word "collaborator": assistant, associate, colleague, confederate, helper, partner, quisling, teammate.
The choice for the best synonym for "collaborator" depends on about whom we are talking about.
For this question, we need to find out how closed Picasso and Braque were:
"Picasso and Braque forged a relationship that was part intimate friendship, part rivalry, and part two-man excursion into the unknown. The two artists were constantly in each other's studio, scrutinizing each other's work while challenging, motivating, and encouraging each other.
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Hope this answer can help you. Have a nice day!