Answer: The correct answer is -
C) Mutualism.
Explanation:
Mutualism is a type long term biological relationship between two organisms belonging to same or different species in which both are benefitted from one another.
For example - E.coli bacteria, present large intestines of human, gets food and place to live in our digestive tract. In return, the bacteria produce vitamin K, which make it harder for other disease causing bacteria to establish in our system.
Thus, option C) is the right answer.
