Which of the following plant cell structures could not be seen when using the 10x objective of the compound microscope?
a) nucleus
2) cell wall
3) cytoplasm
4) endoplasmic reticulum

Endoplasmic Reticulum

ER Golgi apparatus, because it packages proteins received from the ER cytoplasm 

The Golgi body are the ones that slightly alter, organize and prepare so-called parcels to be delivered for all the organelles in the cell. They receive these packages mainly in the rough endoplasmic reticulum. These packages that set out by Golgi body are macromolecules that used and synthesized by cells in many operations. If ER is absent then it would only mean that Golgi body would have no use other than simply lysosomes but these macromolecules plays a dynamic role in many organelles –nutrients, ATP and cell metabolism. It'll have a ripple effect if ER is absent in the cell.