The roots of quadratic equation x 2 – 1 = 0 are :

⇒ x² – 1 = 0

⇒ x² – 1² = 0

⇒ (x + 1)(x - 1) = 0

⇒ (x + 1) = 0 or (x - 1) = 0 [Using zero product rule]

⇒ x = -1 or x = 1.

Hence, option 4 is the correct option.