Which of the following is a common characteristic of ethers?

Ethers are organic compounds characterized by the presence of an oxygen atom bonded to two alkyl or aryl groups, making option C the correct choice. This structural feature gives ethers their distinctive chemical and physical properties, such as their relatively low reactivity compared to other functional groups.

The oxygen atom in ethers serves as a bridge between the two hydrocarbon groups, which can be identical or different. This setup contributes to ethers being generally non-polar and relatively stable molecules. They are often used as solvents in organic chemistry due to their ability to dissolve a wide range of polar and non-polar compounds.

The other options describe characteristics that do not apply to ethers. For example, the presence of a nitrogen atom would indicate a different class of compounds, such as amines. Additionally, while ethers can react under certain conditions, they are not typically classified as reactive with strong acids; in fact, they can be quite resistant to acidic conditions. The mention of phosgene, which is a toxic compound produced from different reactions, is also unrelated to ethers and does not characterize them. Thus, the structural feature described in option C is the defining trait of ethers.