To ensure that potential subject differences are spread out across all experimental conditions, and to conduct sound experimental research, researchers need to use:

Random assignment of participants.

Explanation:

Random assignment is a procedure in which random chances are given to ensure that equal opportunities are given to all the participants in a group of psychological experiments.

Random assignments can be given using methods such as drawing chits, or assigning random numbers to the participants, etc.

For the given scenario also random assignment of participants will be used. Because in this method, the possibility of bias is also eliminated and it also makes the task easy.

So, the correct answer is Random assignment of participants.