D is corrent because sample items should be selected in such a way that the sample can be expected to be representative of the population allowing all items in the population to have an equal opportunity of being selected. A block sample consisting of sequential items may preclude items from meeting the above criteria.
A is incorrect because sequential sampling is an alternative to a fixed sample plan. Instead of examining a single sample of a specified size, the sample is taken in several steps, with each step conditional on the results of the previous step.
B is incorrect because systematic sampling with a random start usually provides a good representation of the entire population because all items in the population have an equal opportunity of being selected.
C is incorrect because populations are often stratified into homogeneous groups to reduce variability in the sample and results in smaller sample sizes.
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