Final answer:
Ceiling effects primarily impact dependent variables by constraining the range of observable outcomes, thus preventing accurate measurement of differences among participants.
Step-by-step explanation:
Ceiling effects can have an impact on both the independent and dependent variables within a research experiment. In relation to the given question, ceiling effects can primarily affect dependent variables. A ceiling effect occurs when the dependent variable being measured has an upper limit, which constrains the range of possible observations to the extent that the true differences in scores among participants cannot be accurately distinguished. This usually results when the measure is not sensitive enough to detect a difference. For example, if all students score close to 100% on an easy test, the test has a ceiling effect because it cannot measure who among the high scorers truly understands the material at a higher level.
The answer to the question would be 'd. dependent variables only.' This is because an independent variable is what is being manipulated or controlled by the researchers to observe its effect on the dependent variable. Therefore, ceiling effects would directly limit the observable impact of the independent variable on the dependent variable, rather than impacting the independent variable itself.