Select the correct algebraic expression.

ab - a - b + 1 = (a - 1) (b - 1)

Put the value of a and b = 1 in all the given equations and satisfy

= ab - a - b + 1 = (a - 1) (b - 1)

1 - 1 - 1 + 1 = (1 - 1) (1 - 1) = 0

= ab + a - b + 1 = (1 - a) (1 - b) (1 - a) (1 + b)

1 + 1 - 1 + 1 = (1 - 1) (1 - 1) (1 - 1) (1 + 1) 

2 ≠ 0

= ab + a + b + 1 = (1 - a) (b - 1)

1 + 1 + 1 + 1 = (1 - 1) (1 - 1)

4 ≠ 0

= ab - a - b + 1 = (a + 1) (1 + b)

1 - 1 - 1 + 1 = (1 + 1) (1 + 1)

0 ≠ 4