A blacksmith uses . . . . . . .

The question asks about the tool a blacksmith uses.
A blacksmith is a person who works with metal, especially iron, by heating it in a fire and shaping it with hammers.
Let's look at the options:
A drill is used to make holes.
A plane is used to smooth wood.
A trowel is used to apply mortar or plaster.
Bellows are used to blow air into a fire to make it burn hotter. Blacksmiths need a very hot fire to work the metal.
Therefore, the correct answer is D: bellows.