The vacuoles are the membranous sacs which are filled with fluid. These spaces inside the vacuoles are free of the cytoplasm and are lined by the membrane. These vacuoles serve the purpose of storage, digestion, ingestion, and excretion. The vacuoles allow the plant cells to attain big size without the accumulation of the bulk.
The vacuoles are present in both plant and the animal cells, however, their size in plant cells is larger. No proteins or microtubules composes these vacuoles.
Hence, the blank can be filled with 'membranous sacs'.