What structure is described as a fluid-filled space in the cytoplasm, especially large in plant cells?

The structure described as a fluid-filled space in the cytoplasm, particularly prominent in plant cells, is indeed a vacuole. Vacuoles serve several essential functions; they store nutrients, waste products, and help maintain turgor pressure, which is crucial for plant rigidity. In plant cells, vacuoles can be quite large and occupy a significant portion of the cell's interior, influencing the overall structure and function of the cell.

Vesicles and ribosomes, while important cellular components, do not share these characteristics. Vesicles are smaller membrane-bound structures that transport materials within the cell, and ribosomes are involved in protein synthesis and do not occupy a significant space in the cytoplasm like vacuoles do. Therefore, the identification of a vacuole is based on its size, function, and fluid-filled nature, making it the correct answer in this context.