2.
Originally, scientists classified
organisms into five kingdoms based,
in part, on structure.
O cell
family
organ

On structure

Explanation:

Robert Whittaker classified the organisms in a broader sense based on many characteristics which could be the limitations of the previous system of classification.

The Whittaker classified the organisms into five large kingdoms called monera, protist, fungi, plants and animals. The basis for this classification is the cell occurrence, their mode of nutrition and cell structure.

In the given question, since the only structure of cell option is provided therefore will be the correct option.

Thus, On structure is correct.