The __________ between Cassius and Brutus was obvious as they struggled to overcome the obstacles in their friendship.

The sentence describes Cassius and Brutus struggling to overcome obstacles in their friendship, which suggests strain or conflict between them. “Tension” means strained relations, so “The tension between Cassius and Brutus was obvious” fits. “Friendship” and “resolution” do not describe the difficulty, and “tone” refers to the way something is expressed.

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