Answer:
D.) They must be representative of the population.
Explanation:
In general a sample collected from a set of naturally distributed data set must also follow the bell curve or natural distribution
In other way round it can be said that sample set must have the same characteristics as that of the main set and hence a sample must be representative of the population.
Option D is correct
I can remember it for my life if I want, or if that person is interesting.
