Step 1: Understand the word.
Mendacious means not telling the truth - dishonest, deceitful, and prone to lying.
Step 2: Find the synonym.
We need a word that conveys dishonesty or creating false impressions.
Step 3: Eliminate and confirm.
Truthful, honest, and straightforward are all antonyms of mendacious. Misleading - giving a false impression - best captures the sense of mendacious among the options.
\[ \boxed{\text{misleading}} \]