a) Titanium dioxide
b) Sodium hydroxide
c) Kaolin clay
d) Calcium carbonate

Answer: b
Explanation: Sodium hydroxide is not a filler used in paper. Calcium carbonate is the most important papermaking filler followed by kaolin clay, with limited use of titanium dioxide. Fillers can improve brightness, opacity, softness, smoothness and ink receptivity, and are essentially insoluble in water under the conditions of use. Filler particles are often 0.5 to 3 mm in diameter, which is much smaller than the fibers used in making paper. Hence, fillers help in making the paper smooth. Not only this, fillers also increase papers’ opacity and brightness as it increases light scattering efficiency.