Final answer:
The relationship between humans and Bacteroides thetaiotaomicron is best described as mutualism, where both organisms benefit from the digestive and metabolic processes involved.
Step-by-step explanation:
The term that best describes the relationship between humans and Bacteroides thetaiotaomicron is mutualism. This is because B. thetaiotaomicron assists in the digestion of complex plant materials that human enzymes cannot process on their own. By doing so, it significantly contributes to human metabolic processes, and in return, it benefits from residing in the nutrient-rich environment of the human gut. This synergistic relationship where both parties benefit is a classic example of mutualism.