The scientist, J. C. Maxwell, developed the theory that small packets of electromagnetic energy called photons produce what?

J. C. Maxwell's contribution to the understanding of electromagnetic energy is foundational to the field of physics. He formulated theories that describe how electric and magnetic fields propagate through space as waves, leading to the concept of electromagnetic radiation. Photons, as identified in quantum theory later on, are the fundamental particles that make up these electromagnetic waves.

When discussing what photons produce, light is the most direct result of electromagnetic radiation. Maxwell's equations describe the behavior of these waves, which encompass a range of frequencies, including visible light. Therefore, the correct answer indicates that photons are responsible for producing light, aligning with the principles of electromagnetism that Maxwell established.

Other options like sound, heat, and shockwaves, while related in various contexts, do not accurately represent the specific output of photons. Sound is a mechanical wave that requires a medium to travel, heat can be a byproduct of various energy interactions but isn’t produced by photons in the context of their primary nature, and shockwaves pertain to disturbances in a medium as well. Thus, light is the clear and correct answer linked directly to the work of J. C. Maxwell and the nature of photons.