Hello again,
I will solve the problems step by step:
\(a - b = sqrt a - sqrt b\)
or, \((sqrt a - sqrt b)*(sqrt a + sqrt b) = sqrt a - sqrt b\) since \(a^2 - b^2 = (a-b)(a+b)\)
or, \((sqrt a + sqrt b) = 1\) this part is crucial since its given a not equal to b, so we can cancel the non-zero term \(sqrt a - sqrt b\) on both sides

or, \(sqrt a = 1- sqrt b\) Now squaring both terms

or,\(a =(1- sqrt b)^2\)
or, \(a =1 + b - 2sqrt b\)

So the given answer is choice C.