(1) The main component of pure cotton hat is natural fiber, the main component of sweater is protein, and the polyester in polyester sweatpants is an organic synthetic substance, so C is selected;

(2)① protein is the nutrient substance rich in fish, and protein is the basic substance of cells and the main raw material for the growth of organisms;

② The human body must take enough calcium every day, and calcium deficiency will lead to osteoporosis;

(3) (1) Carbon monoxide is the main pollution gas produced during gasoline combustion;

② According to the conservation of electrons, the molecular formula of the substance in the equation is 2H2O；; The valence of oxygen in the compound is -2. According to the valence rule, the algebraic sum of valence of each element is zero, and the valence of lead in PbO2 is +4.

So, the answer is: (1) c; ? (2)① protein; ② calcium (ca); ? ③ ① carbon monoxide; ? ②2H2O； +4.