The first thing to analyze in a cell to understand if it is an animal cell is the presence of the nucleus, because animals have eukaryotic cells (with nucleus and membrane-bound organelles). This cell contais nucleus, so it is an eukaryotice cell. However, not only animals have eukaryotic cells, so the presence of a nucleus by itself doesn't mean that it is an animal cell, and there are differences between animal and plant cells (as both are made of eukaryotic cells). Therefore, C is incorrect.
Plant cells will present a vacuole, not animal cells. And we don't see any big vacuoles in this cell. Therefore, A is incorrect as well.
Ribosomes are present in all cells, therefore B is wrong too.
The cell wall is a estructure that stays on the outside of the membrane wall, to protect the cell. This structure would be seen if the cell was a plant cell, not an animal cell. However, we don't see anything surrounding the membrance cell, therefore there is no cell wall. As it is an eukaryotic cell with no cell wall and no vacuole, we can say that it is an animal cell.
Therefore, the correct answer is D. The cell had no cell wall.