Answer:
D. Some biologists have observed Egyptian Plovers, a type of bird, land on the open mouth of a Nile crocodile without being eaten and remove leeches stuck firmly to the crocodile's gums.
Step-by-step explanation:
Mutualism is when two animals are together and they are benefitted and nobody gets harmed. In this situation, the bird and the crocodile aren't attacking each other but they are both helping each other. The bid gets a nice meal and th crocodile can get rid of those leeches. In the other situations there are teo creatures but one ends up getting eaten and that is not what mutualism is about.