Action Spectrum for Photosynthesis is a graphical representation of the effectiveness of different wavelengths of light in driving photosynthetic reactions. It helps in understanding which wavelengths of light are most efficiently absorbed by the photosynthetic pigments in plants or other photosynthetic organisms.

The person who used a prism, green alga Cladophora, and aerobic bacteria to plot the first action spectrum for photosynthesis is Engelmann.

**Julius von Sachs:**
Julius von Sachs was a German botanist who made significant contributions to the field of plant physiology. He conducted experiments on plant nutrition and growth and is known for his work on the physiology of roots and leaves. However, he did not plot the first action spectrum for photosynthesis.

**Melvin Calvin:**
Melvin Calvin was an American biochemist who conducted groundbreaking research on the carbon fixation pathway in photosynthesis, known as the Calvin cycle. He was awarded the Nobel Prize in Chemistry in 1961 for his work. However, he did not plot the first action spectrum for photosynthesis.

**Harold Arnon:**
Harold Arnon was an American biochemist who conducted research on photosynthesis and the role of chlorophyll in the process. He is known for his work on the isolation and characterization of photosystem II. However, he did not plot the first action spectrum for photosynthesis.

**The Correct Answer: Engelmann**
The correct answer is Engelmann.

**Explanation:**
Engelmann, a German botanist, conducted an experiment in 1883 to determine the action spectrum for photosynthesis. He used a prism to separate white light into its component wavelengths and directed the different colors of light onto a filamentous green alga called Cladophora. He also added aerobic bacteria to the experimental setup.

Engelmann observed that the aerobic bacteria congregated in the areas of the algae where the light was most intense. He then plotted the distribution of bacteria along the algal filaments and found that the highest concentration of bacteria was near the blue and red regions of the spectrum. This indicated that these wavelengths of light were most effective in driving photosynthesis.

Engelmann's experiment provided the first evidence for the existence of different pigments in plants that absorb light of specific wavelengths. This laid the foundation for further research on photosynthetic pigments and the understanding of their role in capturing light energy for photosynthesis.

Overall, Engelmann's experiment using a prism, green alga Cladophora, and aerobic bacteria was instrumental in plotting the first action spectrum for photosynthesis and understanding the role of different wavelengths of light in driving the process.