Factorised form of r² -10r + 21 is

⇒ Factorise form will be as follows:

⇒ (r² -10r + 21) = r – (7+3)r + 21

⇒ (r² -10r + 21) = r – 7r – 3r + 21

⇒ (r² -10r + 21) = r(r – 7) – 3(r – 7)

⇒ (r² -10r + 21) = (r – 7)(r – 3)

Option (a) does not match to our solution.

Option (b) does match to our solution.

Option (c) does not match to our solution.

Option (d) does not match to our solution.

So, option (b) is correct answer.