In which of the following organisms regeneration does not occur?

The correct answer is Saccharomyces. Regeneration is the ability of an organism to regrow lost or damaged body parts. 

Planaria, Hydra, and Lizard are all capable of regeneration. 

Planaria can regenerate a whole individual from small body fragments, while Hydra can regenerate tentacles and the body column. Lizards can regenerate their tails. 

Saccharomyces, on the other hand, is a unicellular organism and does not have the ability to regenerate. It reproduces through budding, which is the formation of a new cell from a small outgrowth on the parent cell.