Certain organisms, such as slime molds, can form large plasmodia that contain multiple nuclei without membranes separating the nuclei and cytoplasm into distinct cells. other organisms, like certain flatworms, have tissues that are syncytial, containing multiple nuclei enclosed within one plasma membrane. these organisms can reproduce, interact with their environments, consume nutrients, and produce wastes. based on the tenet of the cell theory that, "all organisms are composed of at least one cell," then:

Presence of cells in slime molds and flatworms support the tenet of cell theory irrespective of presence of multiple nuclei in their cells.

Explanation:

Slime molds and flatworms exhibit all the life processes such as metabolism, reproduction, nutrition, excretion, sensitivity of the surroundings, etc. which in turn make them living beings.

According to the cell theory, cells are the structural and functional units of life. All the organisms are made of one or more cells. New cells are formed by a division in the existing cells.  

Slime molds and flatworms also follow cell theory. As per given information, slime molds and flatworms also have cells as their basic structural units. The presence of more than one nuclei in their cells does not deviate them from the cell theory.