Granum is the correct answer.

Explanation:

The stacks of closely packed thylakoids are called Granum. Granum is a term used to describe the structure of thylakoids in the chloroplasts of plant cells. Thylakoids are membrane-bound compartments within the chloroplasts that contain the pigments and other components necessary for photosynthesis.

The structure of granum is important for the process of photosynthesis as it allows for efficient capture and utilization of light energy. The thylakoids within the granum are arranged in a stacked formation, resembling a stack of coins. This arrangement increases the surface area available for light absorption and enhances the efficiency of photosynthesis.

The granum is surrounded by a fluid-filled space called the stroma, which contains enzymes and other molecules necessary for the dark reactions of photosynthesis. The stroma is where carbon dioxide is converted into glucose during the process of photosynthesis.

The lumen is the inner space within the thylakoid membrane, where the light-dependent reactions of photosynthesis take place. It is filled with a high concentration of protons, which are used to generate ATP, the energy currency of the cell.

The matrix is a term used to describe the innermost compartment of the mitochondria, which is involved in cellular respiration. It is not directly related to the structure of thylakoids or the process of photosynthesis.

In summary, the stacks of closely packed thylakoids in the chloroplasts of plant cells are called granum. The granum plays a crucial role in the efficient capture and utilization of light energy during the process of photosynthesis.