The correct answer is: D. Impervious.
Glazing is a liquid glass that is applied to the surface of earthenware and then fired in a kiln. The glaze melts and fuses to the surface of the earthenware, creating a hard, impervious layer that protects the pottery from water and other liquids.
Option A is incorrect because glazing does not make earthenware hard. In fact, glazing can actually make earthenware more fragile, as the glaze can crack or chip if it is not applied properly.
Option B is incorrect because glazing does not make earthenware soft. In fact, glazing can actually make earthenware more durable, as the glaze protects the pottery from scratches and other damage.
Option C is incorrect because glazing does not make earthenware porous. In fact, glazing can actually make earthenware less porous, as the glaze seals the pores in the pottery.