Sure, let's factorize the given expression step by step:

Given expression: \(1 - (a - b)^2\)

Recognize that this is a difference of squares:

\[1 - (a - b)^2 = 1 - (a - b)(a - b)\]

Use the formula \((a - b)^2 = a^2 - 2ab + b^2\):

\[= 1 - (a^2 - 2ab + b^2)\]

Distribute the negative sign within the parentheses:

\[= 1 - a^2 + 2ab - b^2\]

Rearrange the terms:

\[= (1 - a^2) + 2ab - b^2\]

Factor out \(1 - a^2\) as a difference of squares \(1^2 - a^2\):

\[= (1 - a)(1 + a) + 2ab - b^2\]

Notice that we have a common factor of \(1 + a - b\):

\[= (1 - a + b)(1 + a - b)\]

So, the correct factorization is \((1 + a - b)(1 - a + b)\)