T/f one of the reasons cells are small is size is because of the constraints of diffusion.

True.

Cells need to be small in order to have a high surface area to volume ratio. As a cell grows bigger, its internal volume enlarges and the cell membrane expands. Unfortunately, the volume increases more rapidly than does the surface area, and so the relative amount of surface area available to pass materials to a unit volume of the cell steadily decreases. If cells get too big, the time taken for something in the middle of the cell to travel to the cell membrane would take too long for the cell to function normally.

The answer would be generation