Which of these terms means "of an obscure nature"?

The term "inscrutable" means something that is difficult to understand or interpret, often characterized by an obscure or puzzling nature. This aligns perfectly with the definition of "of an obscure nature," as inscrutable things often leave people confused or unable to see clearly into their meaning or intentions.

The other options have definitions that do not convey the same sense of obscurity. "Insurmountable" refers to something impossible to overcome, "inveterate" describes a habit or commitment that is long-established and unlikely to change, and "indomitable" highlights a strength that cannot be subdued or overcome. None of these terms capture the concept of obscurity like "inscrutable" does.