Which of the following cellular structures is best described as 'closely stacked, flattened sacs'?

Analyze the description: 'Closely stacked, flattened sacs' is a hallmark of the Golgi apparatus. The Golgi apparatus is composed of membrane-bound sacs called cisternae, which are stacked together and play a role in modifying, sorting, and packaging proteins and lipids for secretion or use within the cell.

Eliminate incorrect options: Lysosomes are membrane-bound organelles containing digestive enzymes, not stacked sacs. Mitochondria are double-membraned organelles with a folded inner membrane (cristae), not flattened sacs. Ribosomes are small, non-membranous structures involved in protein synthesis, not stacked sacs.