During smelting, an additional substance is added which combines with impurities to form a fusible product. It is known as:
A.Slag
B.Mud
C.Gangue
D.Flux

Hint: A naturally occurring solid material from which a metal or valuable mineral can be extracted and then be sold at a profit, is known as an ore. Ore is extracted from earth through mining and refining.

Complete answer:
Smelting is a process of applying heat to an ore so that a base metal can be obtained. It is used to extract many metals from their ores; therefore, smelting is a form of extractive metallurgy. Smelting is used to extract silver, iron, copper and other base metals. Smelting uses heat and chemical reducing agents to decompose the ore, leading to remains of metal base as other parts get off as gases or slag. The reducing agents used in smelting are fossil fuels like coke, charcoal. The oxygen in ore binds to carbon at high temperature due to low potential energy in the bonds of carbon dioxide.
The carbon source acts as a chemical reactant to eliminate oxygen from the ore, generating pure metal elements.
 As most ores are impure, it gets necessary to use flux. Flux like limestone is used to remove rock, gangue as well as slag.
Hence flux is an additional substance which is added during smelting, which combines with impurities to form a fusible product.

Therefore, the correct answer is option D.

Note:
Smelting involves more than melting, it has a process which helps in extraction of metal from ore. The processes involved are as follows,
Roasting: In case of sulphates and carbonates, roasting is used. It removes unwanted carbon or sulphur which leaves oxides which can be directly reduced. It is carried out generally in an oxidizing environment.
Reduction: It is the final, high temperature step. In this step, the oxide becomes the elemental metal. A reducing environment is needed for this process.