The incorrect word in the sentence is "wittness" (A), which is a misspelling. The correct spelling should be "witness", meaning someone who sees an event take place, especially in a legal or official context.
• "Wittness" (incorrect) is misspelled; the correct spelling is "witness."
• "Describe" (correct) is used properly as it means to give an account of something in words.
• "Observed" (correct) is correctly used as it means to notice or witness something.
• "Incident" (correct) is used appropriately to refer to an event or occurrence.
• "All are correct" (incorrect) because "wittness" is wrongly spelled.
Hence, option (d) is the answer.