This question examines the understanding of terms related to the act of forming or making up something. The correct answer is 'B. constitute,' which means to form or make up something. Option A, 'separate,' means to divide or keep apart, which is the opposite of forming something. Option C, 'incorporate,' means to include or contain within something else, which is not the same as forming. Option D, 'geologist,' refers to a person who studies geology, which is unrelated to the act of forming or making up something.