The best sample is one that is a systematic sample.

a. convenient.

b. representative of the population.

c. purposefully selected.

d. None of the above.

b. representative of the population.

Explanation:

A sample is a collection of small quantity or part which is normally taken from a large mass of quantity that represents the whole having the same characteristics.

A sample should be a representative of the entire population in consideration to make sure to generalize the results from the sample of the research to the population as a whole. A sample is a subset of the population and is considered as the best sample when it represents the whole population.

Hence the correct option is (b).