Answer:
Experimental research
Step-by-step explanation:
A researcher would use experimental research, if they were interested in determining whether listening to subliminal advertising causes an increase in sales of products.
Experimental researches are used in study the validity of a scientific hypothesis. The scientific hypothesis in this case is that subliminal advertising causes an increase in sales of product.
The null hypothesis would be; listening to subliminal advertising has no effect on sales of product.
The alternative hypothesis would be; listening to subliminal advertising has effect on sales of product.