What does a homologous series of organic compounds have in common?

A homologous series of organic compounds is characterized by having the same functional group. This similarity in functional groups leads to uniform chemical reactivity among the compounds in the series. For instance, in the case of alcohols, all compounds will contain the hydroxyl (-OH) functional group, which governs their chemical behavior, such as how they react with acids or how they interact with other compounds.

While the other options touch upon aspects that might differ among compounds in a series, they do not capture the defining feature of a homologous series. The general formula, while it may vary, is consistent for specific subclasses of compounds. Similarly, chemical properties can vary but are generally predictable within the series due to the same functional group present. Therefore, the defining characteristic that binds all members of a homologous series together is their shared functional group.