I believe the answer would be A, because if water didn’t exist, there would be no presence of living things on earth. Water is a resource for life, thus, presence of living things.
Regarding the order rhizobiales of the class Alphaproteobacteria, the genus that includes human pathogens is Brucella.
Rhizobiales are a type of Alphaproteobacteria, which are a type of Proteobacteria, which are Gram-negative bacteria.
Brucella is also a type of Gram-negative bacteria that can cause brucellosis, which is an infectious disease caused by eating unpasteurized milk or undercooked meat.