In an ecological energy transfer model, such as the one illustrated by the 10% rule, approximately 10% of the energy from one trophic level is transferred to the next trophic level. In this case, when discussing the transfer of energy from producers (like plants) to primary consumers (like herbivores), it is generally accepted that about 10% of the energy stored in the plants (producers) is transferred to the primary consumers.
Therefore, the correct answer is: 10% of the plant's energy is transferred.