Choice A rationale:

Clients aged 20 to 30 years is not the independent variable in this research question. It is likely part of the study's population or sample characteristics but not the variable being manipulated or studied.

Choice B rationale:

This is the correct answer. Antipsychotic-induced weight gain is the independent variable in this research question because it represents the factor or condition that the researcher is manipulating or studying to determine its effect on the outcome variable (pain relief when used in conjunction with oral analgesics).

Choice C rationale:

Educational intervention is not the independent variable in this research question. It is the intervention or treatment being applied to the participants and is more likely the dependent variable, as its effectiveness is being assessed.

Choice D rationale:

Diagnosis of schizophrenia is not the independent variable in this research question. It is a participant characteristic that may influence the study's results but is not the variable being manipulated or studied in this context.