Two plants can be conclusively said to belong to the same species if they can be reproduce freely with each other and form seeds. Species is a natural population of individuals or group of populations which resemble one another in all essential morphological (i.e., body form, size, shape, etc.) and reproductive characters so that they are able to interbreed freely and produce fertile offsprings. A species can be easily distinguished from other closely related species on the basis of their distinct morphological differences
So, the correct answer is 'Can reproduce freely with each other and form seeds'