Concept:This question tests spelling accuracy. One of the given words is written incorrectly.Explanation:Examine each option: Option A 'Perspicacious' is correctly spelled; it means having keen judgment. Option B 'Perspicasious' is wrong – the correct spelling is 'Perspicacious' (the letter 'c' comes after 's'). Option C 'loneliness' is correct, meaning the state of being alone. Option D 'lightning' is correct, referring to an electric flash in a storm. Only option B has a spelling error.Answer:Option B (Perspicasious) is the misspelt word.