Pay close attention to vowel changes when converting adjectives to nouns.
For example, 'pugnacious' becomes 'pugnacity', not 'pugnocity'.
Regular reading of high-level vocabulary lists helps in remembering these subtle changes.
Step 1: Identify the correctly spelled word. We need to find which option follows standard English spelling conventions. Step 2: Analyse each option. Option 1 - perspicacity (keen insight): correct. Option 2 - pugnocity: incorrect; should be pugnacity. Option 3 - sensetivity: incorrect; should be sensitivity. Option 4 - authonticity: incorrect; should be authenticity. Step 3: Confirm. Perspicacity means the ability to notice and understand things clearly - and it is the only correctly spelled word here. \[ \boxed{\text{perspicacity}} \]