Asample in which each member of the universe has an equal chance of being included

This would mean that your sample is representative for the population of the universe. Considering that you picked each person our randomly, that would mean you didn't include any critea on which you based your decisions other than pure randomness. And due to the fact that the choice of members of your sample was random, each had the same probability of being chosen. 

So in short, when each member of the universe has an equal chance of being included, this means your sample is representative :)

Hydrosphere is the correct answer