[A]: The far-red form of the phytochromes ab- sorb light at 740 nm.
[R]: The red form of phytochrome absorbs light at 660 nm

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

### Step-by-Step Solution:

1. **Understanding Phytochromes**:
- Phytochromes are photoreceptor proteins in plants that play a crucial role in regulating various physiological responses to light, including seed germination, flowering, and shade avoidance.

2. **Assertion Analysis**:
- The assertion states that the far-red form of phytochromes absorbs light at 740 nm. This is correct as the far-red form of phytochrome (Pfr) does indeed absorb light at this wavelength.

3. **Reason Analysis**:
- The reason states that the red form of phytochrome absorbs light at 660 nm. This is also correct, as the red form of phytochrome (Pr) absorbs light primarily at this wavelength.

4. **Relationship Between Assertion and Reason**:
- Although both statements (A and R) are true, the reason (R) does not serve as a correct explanation for the assertion (A). The assertion talks about the far-red form, while the reason discusses the red form, indicating that they are related but not directly explanatory.

5. **Conclusion**:
- Both A and R are true, but R is not a correct explanation of A. Thus, the final answer is that both statements are true, but R does not explain A.