A researcher selects a probability sample of 100 out of the total population purely by chance. It is called

Simple random sampling gives every unit of the population an equal and independent chance of selection.

A quota sample fills fixed numbers from categories by the researcher and is a non-probability method.

A stratified random sample first divides the population into strata and then samples within each.

A systematic sample picks every kth unit from an ordered list after a random start.

Selection purely by chance without any grouping is the defining mark of simple random sampling.

Lottery and random-number-table methods are typical ways to draw a simple random sample.