The correct answer is option 'D' - Plasmodesmata.

Explanation:
Plant cells are connected to each other through specialized structures called plasmodesmata. These structures allow for the exchange of various molecules, including nutrients, signaling molecules, and even viruses, between adjacent plant cells.

Let's break down the answer in detail:

1. What are plasmodesmata?
Plasmodesmata are small channels that penetrate the cell walls of adjacent plant cells, connecting their cytoplasms. They are lined with cytoplasmic sleeve and filled with fluid, allowing for the direct exchange of substances between cells.

2. Structure:
Plasmodesmata consist of three main components:
- Desmotubule: A tube-like structure formed by the endoplasmic reticulum that runs through the center of the plasmodesma.
- Plasma membrane: The plasma membrane of each connected cell, which forms the lining of the plasmodesma.
- Cytoplasmic sleeve: A cytoplasmic channel that surrounds the desmotubule and connects the cytoplasms of the two cells.

3. Function:
Plasmodesmata play crucial roles in plant growth, development, and communication. They allow for the transport of molecules such as ions, sugars, amino acids, hormones, and even genetic material (RNA and proteins) between cells. This intercellular communication is essential for coordinating various processes, including tissue differentiation, defense responses, and nutrient sharing.

4. Regulation:
The permeability of plasmodesmata can be regulated to control the flow of molecules between cells. This regulation is achieved through the presence of specialized proteins called plasmodesmata-located proteins (PDLPs) and cytoplasmic strands that connect the desmotubule and plasma membrane. These proteins and strands can either restrict or facilitate the movement of specific molecules.

In conclusion, plasmodesmata are specialized structures that connect the cytoplasms of adjacent plant cells. They play a vital role in intercellular communication and nutrient exchange. Understanding the function and regulation of plasmodesmata is crucial for studying plant development, physiology, and disease resistance.