The correct answer is C. Choosing students who participate in chess club activities is not a technique that can be used to randomly assign students to a treatment group. This is because this method introduces a selection bias, which means that the students in the treatment group are not representative of the population of interest. The other methods, such as selecting a slip of paper from a hat, putting students in alphabetical order and picking every other student, and flipping a coin, are all examples of random assignment techniques, which ensure that each student has an equal chance of being in the treatment group or the control group. Random assignment is important for conducting experiments, because it helps to eliminate confounding variables and increase the validity of the results.