Final answer:
Chemical energy from food is converted into kinetic energy as a person walks and, due to inefficiencies, some is also transformed into thermal energy.
Step-by-step explanation:
When a person is walking on level ground at a constant speed, a transformation of energy is occurring. The energy source in this scenario is the chemical energy stored in the person's body from food they have consumed. As they walk, this energy is converted primarily into kinetic energy — the energy of motion. Additionally, some of the energy is inevitably converted into thermal energy because of the inefficiency inherent in all biological and mechanical processes: no transformation is perfectly efficient, and some energy is always released as heat. Therefore, the correct answer to the question is that chemical energy is being transformed to kinetic energy and thermal energy.