For example whether an organism is unicellular or multicellular would be its _.
There is a word 'cellularity', though I don't think that is quite the word I am looking for.
As a sometime cell biologist, I offer that whether an organism is unicellular or multi-cellular is dependent upon its structure. I apologize if this feels overly simple, and there are other words that would suffice; however, it is certainly accurate to state that:
Respectfully, Dr. Jonas Moses