Hi Lonely_Kie, The second and third statements are correct and both sound natural. The first statement would make sense if we, the readers, knew what "favorites" were. It would be better to write, "Collecting stamps is one of his favorite activities." The last statement has a grammatical error in it. All prepositions must have an object, so "of" needs to be followed by a noun or pronoun. Although "workout" is a noun, it is not an activity. "Working out" is an activity, and adding "-ing" to the end of "work" turns that verb into a gerund, which is the -ing form of a verb used as a noun. In this case, Steven is fond of the activity of working out. I hope this clarifies things. You may have to do some further studying of the difference between participles and gerunds.