Final answer:
The most important virulence factor of Vibrio cholerae is Cholera toxin (d), which disrupts the normal physiology of the intestinal cells, leading to severe diarrhea.
Step-by-step explanation:
The most important virulence factor of Vibrio cholerae is (d) Cholera toxin. Cholera toxin is an enterotoxin that causes severe diarrhea by binding to intestinal epithelial cells and activating an intracellular G protein. This leads to an increase in cyclic AMP, resulting in the secretion of large amounts of fluid and electrolytes into the intestinal tract, which causes the characteristic "rice-water stool" diarrhea seen in cholera. The cholera toxin gene is transferred to V. cholerae through specialized transduction by the CTX phage.