Final answer:
Ethers are formed by the condensation of two alcohol molecules, with dehydration occurring under appropriate conditions to form compounds where two hydrocarbon groups are linked by an oxygen atom.
Step-by-step explanation:
Ethers can be made by the condensation of two alcohol molecules by splitting out a molecule of water. This process is known as a dehydration reaction. For instance, when ethanol is treated with a limited amount of sulfuric acid and heated to 140 °C, diethyl ether and water are formed.
The correct option that fills in the blank is B) alcohol. Under the proper conditions, one alcohol contributes the entire hydroxyl (OH) group whereas only the hydrogen atom from the hydroxyl group of the second alcohol molecule is removed. This leads to the formation of an ether molecule, where two hydrocarbon groups are connected by an oxygen atom.