Ahypothetical poison prevents nucleoli from carrying out their functions.

If a hypothetical poison prevent nucleoli from carrying out their functions, the cell organele that will be affected is the RIBOSOMES.
Nucleoli of the cell is the one that is responsible for production of ribosomal sub units; it uses proteins and ribosomal RNA to do this. It then send the sub units to the interior of the cell where they are assembled.
Ribosomes make proteins in the cells. Thus, if nucleoli is affected, ribosome will not be produced and ultimately proteins will not be produced in the cells.

Fungi is were it belongs