Answer: Causal
Explanation: Causal research refers the research which focuses on analyzing the cause and effect relationship. In such researches the researcher tries to observe the change in main variable that is causing the change in other variables.
In the given case, Vernon is changing the price of the product to ascertain the impact of it on the sales made. Thus, they they are trying to study the relationship between price and sales.
Hence from the above we can conclude that the correct option is C.