In Engelmann's experiment, bacteria were used to

Options

A. split light into its spectral components.
B. detect the sites of `O_2` evolution.
C. absorb the spectra of chlorophyll is a and b using Cladophora.
D. detect the sites of `CO_2` evolution and monitor the algal photosynthesis.

Answer

### Step-by-Step Solution:

1. **Understanding Engelmann's Experiment**: Engelmann's experiment aimed to investigate the relationship between light and photosynthesis. He used a green alga called Cladophora to study how different wavelengths of light affect the process of photosynthesis.

2. **Use of Glass Prism**: Engelmann utilized a glass prism to split white light into its spectral components (colors). This allowed him to illuminate the Cladophora with different wavelengths of light.

3. **Introduction of Bacteria**: He introduced aerobic bacteria (which require oxygen) to the setup. The bacteria were used as indicators of oxygen production because they would accumulate in areas where oxygen was being produced during photosynthesis.

4. **Observation of Bacterial Accumulation**: Engelmann observed that the bacteria accumulated in areas illuminated by blue and red light. This indicated that these wavelengths were most effective for photosynthesis, leading to higher oxygen production.

5. **Conclusion Drawn**: From the experiment, Engelmann concluded that the blue and red light regions provided sufficient energy for photosynthesis, as evidenced by the accumulation of bacteria in those areas. This accumulation indicated that oxygen was being produced in those regions.

6. **Final Answer**: Therefore, the correct answer to the question "In Engelmann's experiment, bacteria were used to" is **B. detect the sites of oxygen evolution**.