Final answer:
The best answer choice for an antigen that is present in some but not all members of a species is not clearly presented among the options given. However, based on the descriptions, a hapten can become antigenic when combined with a carrier molecule, although it typically is not specific to just some members of a species.
Step-by-step explanation:
An antigen is a macromolecule that can elicit an immune response. The part of the antigen that is specifically recognized by the immune system is known as an epitope. An antigen that is present in some but not all members of a species is known as a private antigen, which is not one of the options provided. Based on the context provided, the best answer would be a hapten, which can become antigenic when attached to a larger carrier molecule. However, none of the options (ubiquitous antigen, common antigen, hapten, or epitope) perfectly fits the description of an antigen present only in some members of a species. Ubiquitous antigens are found in all members of a species, common antigens might be shared by many but not necessarily exhibiting specificity within a species, haptens are small non-antigenic molecules until they bind to a carrier, and epitopes are specific parts of an antigen recognized by antibodies.