In the following sum, '?' stands for which digit : ? + 1? + 2? + ?3 + ?1 = 21?

Answer: Option C

Let the required digit be x
Then,
⇒ x + 1x + 2x + x3 + x1 = 21x
⇒ x + 10 + x + 20 + x + 10x + 3 + 10x + 1 = 200 + 10 + x
⇒ 22x = 210 - 34 = 176
⇒ x = 8
Hence, the required digit is 8