The correct answer is - share identical genetic characteristics.
A species can be defined as a group of organisms that are sharing the same genetic characteristics, which are different from the other organisms and set them apart from them as unique. The members of a certain species can produce offspring, but the problem with using this as a defining trait is that there's lots of cases where multiple different species can produce offspring between them, so that can come as very confusing and merge multiple species as one (margay and south tigrina for example). Also, the organisms that are grouped as a separate species, do look exactly alike, but the evolution is managing to create organisms in different parts of the world, of totally different ancestry, to look alike, because of the parallel evolution in accordance to the living conditions (Thylacosmilus and Barbourofelis for example).
