Here's why:
* Commensalism is a type of symbiotic relationship where one organism benefits and the other is neither harmed nor helped. In this case, the E. coli benefit by getting a place to live and access to nutrients, while we benefit from the vitamin K they produce.
* Mutualism would describe a relationship where both organisms benefit. While we do benefit from the vitamin K, it's not clear if the E. coli gain any significant direct benefit beyond a comfortable environment.
* Parasitism would describe a relationship where one organism benefits at the expense of the other. This is not the case with E. coli in the human gut.
Therefore, commensalism is the most accurate term for the relationship between humans and the E. coli in our gut.
