The effect on the melting point of a pure substance when another substance is added depends on the nature and quantity of the added substance. Typically, the presence of impurities lowers the melting point of the pure substance. This occurs because impurities disrupt the regular arrangement of particles in the solid, making it easier for the substance to overcome intermolecular forces and transition into the liquid phase. Thus, the correct answer is option [B] Decreases. In some cases, if the added substance forms a compound with the pure substance, it may initially increase the melting point due to stronger intermolecular forces, but eventually, as more impurities are added, the melting point decreases. However, in most scenarios, the presence of impurities leads to a decrease in the melting point of the pure substance.