Simple random sampling:

• This technique refers to be the fundamental techniques that belong to probability sampling methods.
• In this technique, each of the group individuals has the same chances to get selected in the sample so that sample turns to be biased free.

In the given scenario, the total number of individuals in the population is 6 million. Here, the researchers want to select only 500,000 individuals so that there is an equal chance of representation for each individual. Therefore, the best sampling strategy to be applied in this case is simple random sampling.