KBC 2021 | What is the letter ‘X’ denote in X-ray? KBC 2021, Check the correct answer to the question - What is the letter 'X' denote in X-ray? By Chirag Gaur - September 8, 2021 KBC 2021, The options for the question – What is the letter ‘X’ denote in X-ray? Xenon Unknown Illegal Radiation The correct answer is Unknown.