A. Use alum to purify drinking water. The aluminum hydroxide colloid generated by the hydrolysis of aluminum ions has an adsorption effect, which is related to the properties of the colloid. Therefore, A is not selected;

B. Use Making tofu with gypsum or salt brine utilizes the coagulation of colloids, which is related to the properties of the colloids. Therefore, B is not selected;

C. When NaOH solution is added dropwise to FeCl3 solution, a reddish-brown precipitate appears. What occurs is a metathesis reaction and has nothing to do with colloids, so C is selected;

D. The beautiful scene produced by the early morning sunlight passing through the dense forest branches and leaves is the Tyndall phenomenon of colloids, which is related to colloids, so D is not selected;

So C.