The correct answer is option a, that is, leaf to mouse.
In a food pyramid, the species at the lower level of the food chain directly obtain their energy from the Sun and are considered as the primary producers. This level mainly comprises the plants. The species that are on the next levels are the secondary and tertiary consumers respectively. The level of the energy conversion is the greatest at the lowest trophic level and as one moves up the level of energy conversion gets lower.
In the given choices, there is a need to determine which of the changes are at the lowest trophic levels. Of these, the option leaf to the mouse is at the lowest level since plant or leaf is a primary producer, and hence, the level of the energy conversion is the greatest in comparison to the other options.