A plant cell
Explanation:
In the given question, the characteristics of a cell are provided which includes:
1. Rectangular shaped cells which are rigid: Rigidity of a cell is provided by the cell wall which is a characteristic feature of the plant cell.
2. Possess a nucleus: which shows that the organism is a eukaryote
3. The oval-shaped green structures: the oval-shaped structure which can move is the chloroplast which provides the site for photosynthesis.
Thus, it is confirmed that the observed cell is of a plant cell.