The calculated question type was built to test if the user can calculate a given numerical value from a mathematical formula that is not necessarily given in the question text i.e. circle area from the radius.
The unit parameter was added to allow the user to express the numerical value in various valid units .
This is why you need to specify a valid numerical factor for each unit to convert the numerical result in a correct answer. For the circle area each unit should be a "square" unit.( cm2, m2, in2,ft2)
The default unit with a 1 factor is defined following the calculation done in the equation you use to calculate the answer.
You can set your formula so that it calculates in m2 although the radius is expressed in the question text in cm.
Given this, the natural way to express the units is NOT randomize but natural like mm2 , cm2, m2.
If you want to test in which unit you can express a given physical quantity then a multiple choice question will be the right way to do it.