a) The traits in the plant population randomly changed, and would have done so regardless of an increase in rainfall.
b) Due to the increased sea level, mutations occurred that led to some of the plants being shorter with thicker leaves, allowing them to withstand submergence at high tide.
c) Each plant evolved to become shorter with thicker leaves in order to thrive in this wetter environment. They then passed these traits on to their offspring.
d) Shorter plants with thicker leaves that already existed in the population were better able to thrive and generate offspring under the new conditions.