The correct answer will be- multiple observations.
Explanation:
The organism is composed of the cell was suggested by the Schleiden and Schwann in 1839. They observed that both plant and animals are made up of structures which contains a nucleus. This observation provided evidence that the organism is made up of cells.
The earlier observations made up by Robert Hooke when he observed cork cell and Anton van Leeuwenhoek when he observed the algae Spirogyra also provided the evidence.
The development of theory is based on the observations made by the scientist like Robert Hooke, Anton van Leeuwenhoek and Schleiden and Schwann and not controlled experiments which proves that organisms are made up of cells.
Thus, multiple observations are the correct answer.
The generalization that all organisms are composed of cells was established through multiple observations in various kinds of animals and plants. A controlled experiment of finding cells in all the kinds of living organisms is not possible or is infeasible as it will require prior knowledge of all the kinds of organisms inhabiting Earth and the effort to catch and bring them in the controlled environment of the lab for testing.
