Recall that variability is a way of measuring how different are a set of numerical values from each other. So, in this case, we want to choose a question to which the answers from the sample are not different from each other. We are told that all apartments are identical, so if the question is referred to the characteristics of the apartments, we will get the same response.
From the given options, we can see that the only question referring to the characteristics of the apartments is how many bedrooms are in your apartment ?. Note that as the apartments are identical, the answer to this question would be the same number (let's say 3 as an example). So the variability of this set would be 0 as we only have one numerical value in the set.