The number of elements of the set {x: x ∈ Z, x² = 1}is

The set defined by {x: x ∈ Z, x² = 1}

This means the elements of the set is given by x² = 1 and that x is belongs to integers(Z)

x = 1 ⇒ x² = 1

x = -1 ⇒ x² = (-1)² = 1

For the x = 1 & -1 the statement holds true.

∴ 1, -1 are the elements of the set

The number of elements in the set is 2.

Option B is the correct answer.