Which of the following describes a sample that is randomly assigned?

The correct answer describes a process where each participant in a study has an equal opportunity to be chosen for the sample. This characteristic is essential for random assignment, as it helps eliminate biases that can occur when certain participants are favored or excluded. By ensuring that every individual has an equal chance to be selected, researchers can better generalize their findings to the larger population and strengthen the validity of their conclusions. Random assignment contributes to the integrity of experimental designs, as it aids in creating equivalent groups that can be compared reliably under controlled conditions.

The other descriptions do not meet the criteria for random assignment: allowing participants to choose their own groups can lead to self-selection bias; basing assignments on prior knowledge introduces confounding variables that may affect outcomes; and including only some participants does not represent the larger population adequately, thus undermining the randomness necessary for robust scientific conclusions.