Select the odd word/letters/number/number pair from the given alternatives.

Let's analyze the meanings of the given words:
Adamant, Stubborn, and Tenacious are all synonyms that describe a person who is unyielding, firm, or resolute in their stance or opinions.

Lenient, on the other hand, means permissive, merciful, or tolerant, which is completely opposite in nature.
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Therefore, "Lenient" is the odd one out.