The correct option is D. contain chloroplast. Detailed Explanation:

1. Understanding Guard Cells:
2. Guard cells are specialized cells located in the epidermis of plant leaves, particularly in dicots (plants with two seed leaves). Their primary function is to regulate the opening and closing of stomata, which are small pores on the leaf surface that allow for gas exchange (CO2 in, O2 out) and transpiration (water vapor loss).
3. Chloroplast Presence:
4. One of the key features that differentiate guard cells from other epidermal cells is that guard cells contain chloroplasts. Chloroplasts are the organelles responsible for photosynthesis, the process by which plants convert light energy into chemical energy. The presence of chloroplasts in guard cells allows them to produce ATP and other energy-rich molecules that are necessary for the active transport processes involved in stomatal movement.
5. Mechanism of Stomatal Movement:
6. When guard cells take up potassium ions (K+), water follows by osmosis, causing the cells to swell and become turgid. This turgidity causes the stomata to open. Conversely, when potassium ions are released, water exits the cells, leading to a loss of turgor pressure and the closing of the stomata. The energy required for these processes is partly derived from the chloroplasts within the guard cells.

Why the Other Options Are Incorrect:

• A. Have no definite shape:
• This statement is misleading. While guard cells can change shape as they swell or lose water, they do have a definite shape when turgid (curved). Other epidermal cells also have defined shapes, so this option does not accurately describe a unique characteristic of guard cells.
• B. Lack nuclei:
• This is incorrect. Guard cells, like most living plant cells, contain nuclei. The nucleus is essential for regulating cellular activities, including the synthesis of proteins and other molecules necessary for the guard cells' function. Other epidermal cells also have nuclei, so this does not distinguish guard cells.
• C. Are smaller:
• Guard cells are not necessarily smaller than other epidermal cells. In fact, they can be similar in size to other epidermal cells. The size of guard cells can vary depending on the plant species and environmental conditions, but this characteristic does not serve to differentiate them from other epidermal cells.

Summary of Key Points:

• Guard cells contain chloroplasts, which are essential for energy production during stomatal movement.
• They regulate gas exchange and transpiration by changing shape in response to water and ion movement.
• Other epidermal cells do not have chloroplasts and do not perform the same functions as guard cells.
• Understanding the unique features of guard cells is crucial for grasping how plants manage water loss and gas exchange.

This thorough understanding of guard cells and their unique characteristics will help you in your studies of plant biology and physiology.