Step 1: What is asked.
We must name the scientist who first described the action spectrum of photosynthesis.
Step 2: Meaning of action spectrum.
An action spectrum is a graph that shows how fast photosynthesis goes at each colour, or wavelength, of light.
Step 3: The classic experiment.
Engelmann split light into its colours with a prism and shone it on the green alga Cladophora. He added oxygen-loving bacteria to the slide.
Step 4: What he saw.
The bacteria gathered most in the blue and red light regions, because those colours gave the most photosynthesis and the most oxygen. This gave the first action spectrum.
Step 5: Why not the others.
Sachs showed starch forms in the green parts of leaves. Priestley discovered that plants release oxygen. Van Niel explained that oxygen in photosynthesis comes from water. None of them drew the first action spectrum.
Step 6: Conclusion.
Therefore the correct answer is "Engelmann". \[ \boxed{\text{Engelmann}} \]