The correct answer is option 'A' - Certain symbiotic microorganisms normally occur in the caecum in the human body. Let's explore this answer in detail:

Symbiotic microorganisms are microorganisms that live in a mutually beneficial relationship with their host. They provide various benefits to the host, such as aiding in digestion, protecting against harmful pathogens, and supporting the immune system.

- The Caecum: The caecum is a pouch-like structure located at the beginning of the large intestine. It is a common site for symbiotic microorganisms to reside in the human body. The caecum provides an ideal environment for these microorganisms to thrive due to its warm and moist conditions.

- Symbiotic Microorganisms: The symbiotic microorganisms that commonly inhabit the caecum include bacteria, archaea, and fungi. These microorganisms play a crucial role in the breakdown of complex carbohydrates, such as dietary fiber, which cannot be digested by human enzymes alone. They produce enzymes that can break down these carbohydrates into simpler forms that can be absorbed by the body.

- Digestive Function: The symbiotic microorganisms in the caecum produce enzymes, such as cellulases and hemicellulases, that help in the digestion of dietary fiber. This process is known as fermentation, and it results in the production of short-chain fatty acids, such as acetate, propionate, and butyrate. These fatty acids are an important energy source for the cells lining the colon and also play a role in maintaining a healthy gut environment.

- Immune Function: The symbiotic microorganisms in the caecum also play a crucial role in supporting the immune system. They help in the development and maturation of the immune system by interacting with immune cells in the gut-associated lymphoid tissue (GALT). Additionally, they compete with harmful pathogens for resources and space, thereby protecting the host from infections.

- Gut Microbiota: The collection of symbiotic microorganisms in the caecum, along with those in other parts of the digestive tract, is collectively known as the gut microbiota. The gut microbiota is highly diverse and unique to each individual. It can be influenced by factors such as diet, age, genetics, and environmental exposures.

In conclusion, certain symbiotic microorganisms are commonly found in the caecum of the human body. These microorganisms play important roles in digestion, immune function, and overall gut health.