The answer is the square root of 50.
This is because it’s the only nonreal number. Real numbers can be expressed as fractions. This means that whole numbers, decimals, fractions, and any number that can be ultimately put as a fraction is a real number.
50/2 is already a fraction, so it’s real. 50.1 can also be made in to a fraction. 50.1 (repeating) has a pattern to its decimal values, meaning it is also able to be a fraction. However, the square root of 50 is a number with decimal values that go on forever and never have a pattern to it, and thus cannot be put into a fraction. Thus, the square root of 50 is not a real number, and so is the odd one out.