* Amoebas are eukaryotic organisms. This means they have a true nucleus and other membrane-bound organelles within their cells. Prokaryotes, like bacteria, lack these structures.

Let's look at the other options:

* A. asexual: Amoebas reproduce asexually, but so do many animals (like some sea anemones). This isn't a defining characteristic of non-animals.

* B. unicellular: Amoebas are unicellular, but so are many other organisms that aren't considered animals (like algae and bacteria).

* D. autotrophic: While some amoebas can be autotrophic (making their own food), many are heterotrophic (consuming other organisms). Animals are always heterotrophic.

Therefore, the most fundamental reason why an amoeba isn't an animal is its eukaryotic cell structure. Animals are multicellular organisms made of eukaryotic cells.