The question pertains to the identification of the largest structure within the Limbic System. The options provided are:

• Hippocampus
• Cingulate gyrus
• Fornix
• Amygdala

The Limbic System is a complex set of structures in the brain, which supports a variety of functions including emotion, behavior, motivation, long-term memory, and olfaction.

To determine the largest structure among the options, let us examine each:

1. Hippocampus: This structure is primarily involved in memory formation and spatial navigation. It is not the largest in volume among the structures listed.
2. Cingulate gyrus: This is an extensive region of the brain situated above the corpus callosum. It plays a significant role in emotion formation and processing, learning, and memory. It is indeed large and extends across the medial cerebral cortex.
3. Fornix: This is a C-shaped bundle of nerve fibers that functions as the major output tract of the hippocampus. It is not among the largest structures in terms of volume.
4. Amygdala: Known for its role in emotion and behavior, particularly fear and pleasure responses, the amygdala is not the largest structure in the Limbic System.

Based on this explanation, the largest structure in the Limbic System among the choices given is the Cingulate gyrus.

Thus, the correct answer is:

Cingulate gyrus