Thinking about this through a number line would make the solution a lot simpler. 

We can eliminate option B and C because we are talking about the distance between two numbers, that would be an absolute value and thus never less than zero. 
Option A comes close but miss out on the case of zero. The distance between zero and double of zero, which would be zero again is also zero. 
That is why Option D would be the correct answer. 
It covers all the cases as the distance between a number and double of that number, for negative numbers, positive numbers and zero.