A: The first photosynthetic organisms which show oxygenic photosynthesis were cyanobacteria.
R: Photosynthesis by Eubacteria is Anoxygenic.

Options

A. If both As and R are correct and R is correct explanation of As
B. If both As and R are correct but R is not correct explanation of As
C. If As is correct, R is wrong
D. If both As and R are wrong.

Answer

To solve the assertion and reason question, we will analyze both statements step by step.

### Step 1: Analyze the Assertion (A)
- The assertion states: "The first photosynthetic organisms which show oxygenic photosynthesis were cyanobacteria."
- **Explanation**: This statement is true. Cyanobacteria are indeed the first organisms known to perform oxygenic photosynthesis, which means they use water (H2O) as a source of electrons and produce oxygen (O2) as a byproduct.

### Step 2: Analyze the Reason (R)
- The reason states: "Photosynthesis by Eubacteria is Anoxygenic."
- **Explanation**: This statement is misleading. While it is true that some Eubacteria (specifically certain groups like purple and green sulfur bacteria) perform anoxygenic photosynthesis, it is incorrect to generalize that all Eubacteria perform anoxygenic photosynthesis. Cyanobacteria, which are classified under Eubacteria, perform oxygenic photosynthesis.

### Step 3: Conclusion
- From the analysis, we can conclude:
- Assertion (A) is true.
- Reason (R) is false because it incorrectly generalizes the type of photosynthesis performed by Eubacteria.

### Final Answer
- The correct answer is that the assertion is true, but the reason is false. Therefore, the answer is option three.

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