Answer : Option B) A boy brought a pair home as pets.
Explanation : As snails are very small organisms which can hardly travel distances. So, it is not valid that the snails swam across the ocean.
If it got stuck on an airplane's wing it might get pushed away by the high pressure of air during flight.
The most legitimate and probable answer seems to be that, A boy who had visited Africa purchases the pair as pets and brought them to Florida