"Right in rem" refers to a right enforceable against the whole world. It relates to ownership and possession of property.
For example, an owner's right to exclude everyone else from using his land is a right in rem.
This is distinct from "right in personam", which is enforceable only against a specific person (e.g., a contractual obligation).
"Locus standi" means the right to approach the court, and "Fundamental Right" is a constitutional guarantee — neither directly answers the question.