Before making a decision, the leader considered all ...... of the argument.sidesfeaturesperspectivesshades

The most appropriate word to complete the sentence is "perspectives."

Explanation

In the context of making a decision, it's crucial for a leader to analyze various viewpoints or "perspectives" regarding an argument. This allows the leader to understand different angles and implications of the issue at hand, ensuring a more thorough evaluation.

Considering "sides," while closely related, typically refers to opposing viewpoints without the nuance that "perspectives" provides. "Features" and "shades" do not fit well in this context as they don't convey the idea of differing viewpoints or considerations that are vital in decision-making processes.

Therefore, the completed sentence reads: "Before making a decision, the leader considered all perspectives of the argument." This emphasizes the importance of a well-rounded analysis when faced with complex decisions.