I am sometimes attacked for imposing 'rules'. Nothing could be further from the truth. I hate rules. All I do is report on how consumers react to different stimuli. I may say to a copywriter, "Research shows that commercials with celebrities are below average in persuading people to buy products. Are you sure you want to use a celebrity"? Call that a rule? Or I may say to an art director, "Research suggests that if you set the copy in black type on a white background, more people will read it than if you set it in white type on a black background."__________
If we look at the flow of the paragraph, option (c) seems to fit in best because in the last line, the author is talking about the suggestion he would give to an art director. Option (a) is also pretty close but nowhere in the paragraph is the author trying to give any guidance. He is only talking of suggestions