A student puts a drop of reaction mixture of a saponification reaction first on a blue litmus paper and then on a red litmus paper. He may observe that:

Saponification is the process of preparing soap by the hydrolysis of fats or oils with a strong base like sodium hydroxide ($NaOH$). The resulting reaction mixture contains soap (which is a salt of a weak fatty acid and a strong base) and leftover alkali, making the mixture basic/alkaline in nature. Basic solutions turn red litmus paper blue and have no effect on blue litmus paper. Therefore, option (c) is correct. Option (a) is incorrect because red litmus does not turn white. Option (b) is incorrect because bases do not turn blue litmus red. Option (d) is incorrect because a distinct colour change is observed on the red litmus paper.