D

Step 1: The photoelectric effect refers to the emission of electrons from a material when it is exposed to light.
Step 2: Heinrich Hertz was the first to observe the photoelectric effect in 1887, but he did not provide a theoretical explanation for it.
Step 3: Max Planck contributed to the understanding of quantum theory, but he was not the one who explained the photoelectric effect.
Step 4: The correct answer is Albert Einstein, who, in 1905, explained the photoelectric effect by proposing that light consists of packets of energy called photons.
Step 5: Einstein introduced the concept that the energy of these photons is directly proportional to their frequency, leading to the equation E = hν, where h is Planck's constant and ν is the frequency of light.
Therefore, the correct answer is D.