Hi!
Good eye - "guarantees" is why the question is wrong. Randomization makes it more likely that a sample is representative but (because it's random) it can't guarantee anything. You could randomly select a sample that is slightly older, or has different social influences than the general population. The reason to use randomization, as C suggests, is to avoid doing this, but as with any random process there is a chance representatives of certain parts of a population won't be selected. There are also other factors that influence representativeness. For example, you could have a sample that is too small & can't include enough representatives - regardless of randomization. Hope this helps!