In sampling methods, when each and every unit in a population has an equal chance of being selected, it is called:

Correct option is D

Simple random sampling ensures each unit has equal probability of selection, reducing selection bias.
Explanation of options: (a) ✘ Stratified = population divided into strata, then sampled. (b) ✘ Cluster = random selection of groups, not every unit equal chance. (c) ✘ Systematic = every kth unit chosen, not equal for all. (d) ✔ Correct; simple random = all units equal chance.