Which term describes someone who is argumentative or difficult to deal with?

The term that describes someone who is argumentative or difficult to deal with is "contentious." This word is used to characterize individuals who tend to engage in disputes or provoke disagreement. A contentious person is often seen as combative or prone to controversy, which aligns well with the idea of being difficult to deal with in discussions or negotiations.

In contrast, the other terms describe different qualities. "Tranquil" refers to being calm and peaceful, which contradicts the notion of being argumentative. "Affable" describes a friendly and easygoing demeanor, suggesting that the person is approachable and agreeable, rather than difficult. "Complacent" usually refers to a sense of self-satisfaction or smugness, often without awareness of potential dangers or deficiencies, which does not align with the definition of someone who is argumentative. Thus, "contentious" is the most fitting choice for the described behavior.