Final answer:
Mary the fish died due to asphyxiation after the water in her fishbowl was spilled out when John the cat knocked it over.
Step-by-step explanation:
When John the cat knocked over the fishbowl, causing the water to spill out and Mary the fish to die, the correct answer to what happened to Mary is none of the provided options except by inference that Mary has passed away due to a lack of water.
Fish require water to breathe, and without it, they cannot get the oxygen they need. Hence, they are unable to survive, which is commonly referred to as death due to asphyxiation, not drowning.
The other options, such as escaping, being eaten, or turning into a mythical creature, cannot be substantiated without additional context that points specifically to those outcomes.