Several scientists, at various point of time were involved with the discovery of cell and development of modern cell theory. Prior to Robert Hooke, Jan Swammerdam had seen red blood corpuscles of frog. Marcello Malpighi also observed small structures or utricles in slices of plants and animal tissues. Nehmiah Grew initiate cell concept when he found that plant tissues contain minute elementary structures or cells. In 1838, German botanist Matthias Schleiden found that all plant tissues are made up of cells which have essentially similar structure. Thus, it was a 'basic generalization' extended to all the plants by Schleiden. Schleiden by himself neither discovered cell nor described their structure but extended the cell concept to all plants. Hence, he provided a generalization not a hypothesis or law.
So, the correct answer is option C.