Square of 9x – 7xy is

⇒ Applying formula

⇒ (a – b)² = a² + b² – 2ab

⇒ square of (9x – 7xy) means (9x – 7xy)²

⇒ Here, a = 9x and b = 7xy

⇒ (9x – 7xy)² = (9x) + (7xy) – 2×(9x)×(7xy)

⇒ (9x – 7xy)² = 81x² + 49x²y² – 126x²y

Option (a) does not match to our solution.

Option (b) does not match to our solution.

Option (c) does match to our solution.

Option (d) does not match to our solution.

So, option (c) is correct answer.