If ab = 1, where a and b are real numbers then?     a = b-1      b = a     a = b = 2     none of the mentioned

The correct answer is "none of the mentioned".

Here's why:

Given that ab = 1, we can't directly infer any of the provided options.

Option a: a = b-1. If we substitute this into the equation, we get (b-1)b = 1, which simplifies to b^2 - b - 1 = 0. This doesn't necessarily hold true for all real numbers a and b.

Option b: b = a. If we substitute this into the equation, we get a*a = 1, which simplifies to a^2 = 1. This means that a could be 1 or -1, not any real number.

Option c: a = b = 2. If we substitute this into the equation, we get 2*2 = 1, which is not true.

Therefore, none of the mentioned options are correct.