The parasympathetic nervous system is a division of the autonomic nervous system and is responsible for controlling involuntary actions such as respiration, heart rate, and gland secretion. It works in opposition to the sympathetic nervous system, promoting relaxation, comfort, and conservation of energy.

The correct answer is option 'D', the Parasympathetic Nervous System.

The nervous system is divided into two main divisions: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The PNS is further divided into two branches: the Sympathetic Nervous System (SNS) and the Parasympathetic Nervous System (PNS).

The Parasympathetic Nervous System is responsible for controlling involuntary actions like respiration, heart rate, and gland secretion. Let's understand how it functions and its role in maintaining homeostasis in the body.

1. Parasympathetic Nervous System (PNS):
The PNS is often referred to as the "rest and digest" division of the autonomic nervous system. It is responsible for maintaining normal body functions during periods of rest and relaxation. The PNS acts in opposition to the SNS, which is responsible for the "fight or flight" response during stressful situations.

2. Role in Controlling Involuntary Actions:
The PNS controls various involuntary actions that are essential for the body's normal functioning. Some of these actions include:
- Respiration: The PNS regulates the rate and depth of breathing. It stimulates the smooth muscles in the bronchioles to constrict, leading to a decrease in the diameter of the airways and a decrease in respiratory rate.
- Heart Rate: The PNS helps to slow down the heart rate by sending signals to the sinoatrial (SA) node, which is responsible for initiating the electrical impulses that regulate heart contractions. It decreases the heart rate to maintain a normal resting heart rate.
- Gland Secretion: The PNS stimulates the secretion of various glands in the body, including salivary glands, digestive glands, and lacrimal glands. It helps in the production of saliva, digestive enzymes, and tears, which aid in the process of digestion and maintaining healthy eyes.

3. Neurotransmitter Involved:
The PNS releases a neurotransmitter called acetylcholine, which binds to specific receptors on target cells and triggers a response. Acetylcholine acts as the primary neurotransmitter in the PNS and is responsible for transmitting signals between nerve cells.

4. Maintaining Homeostasis:
The PNS plays a crucial role in maintaining homeostasis, which is the body's ability to maintain a stable internal environment despite external changes. By regulating respiration, heart rate, and gland secretion, the PNS helps to maintain the balance of essential physiological processes in the body.

In conclusion, the Parasympathetic Nervous System (PNS) is responsible for controlling involuntary actions such as respiration, heart rate, and gland secretion. It functions in opposition to the Sympathetic Nervous System (SNS) and helps maintain homeostasis in the body.