Final answer:
A plasmodesma is the ring of cytoplasm that facilitates intercellular communication and transport of materials between adjacent plant cells through channels called plasmodesmata. These structures are essential for plants to transfer nutrients, and water, and to coordinate responses, but are not found in animal cells, where gap junctions perform a similar function.
Step-by-step explanation:
A plasmodesma is the ring of cytoplasm that passes through a plasmodesmata, which are channels between the cell walls of two adjacent plant cells. These channels allow materials such as water, nutrients, and signaling molecules to pass directly from the cytoplasm of one plant cell to the cytoplasm of another, facilitating intercellular communication and transport of substances. Contrary to information C, which incorrectly states that plasmodesmata are essential in animal cells and gap junctions in plant cells, it is actually the other way around; plasmodesmata are found in plant cells while gap junctions occur in animal cells.
In general, the plasma membranes of neighboring plant cells cannot touch each other due to the presence of cell walls. However, the presence of plasmodesmata overcomes this barrier and creates a continuous network throughout the plant. This network is vital for the plant's physiology, allowing the seamless transfer of essential solutes or water from roots to leaves, as well as enabling plants to respond to environmental stimuli in a coordinated manner.