Which of the following statements is correct

Options

A. Stems are usually unbranched in both Cycas and Cedrus
B. Horsetails are gymnosperms
C. Selaginella is heterosporous, while Salvinia is homosporous
D. Ovules are not enclosed by ovary wall in gymnosperms

Explanation

To determine which of the statements is correct, we will analyze each statement regarding gymnosperms and their characteristics.

### Step-by-Step Solution:

1. **Analyze the First Statement**:
- The statement claims that "Stems are usually unbranched in both cycad and cedar plants."
- **Evaluation**: Cycads typically have unbranched stems, but cedars (which are conifers) can have branched stems. Therefore, this statement is incorrect.

2. **Analyze the Second Statement**:
- The statement claims that "Horsetails are gymnosperms."
- **Evaluation**: Horsetails belong to the group called Pteridophytes, specifically the genus Equisetum, and they do not produce seeds. Therefore, this statement is incorrect.

3. **Analyze the Third Statement**:
- The statement claims that "Salvinia is heterosporous while Salvinia is homosporous."
- **Evaluation**: Salvinia is indeed a heterosporous plant, while Salvinia is also heterosporous. This statement is incorrect as it contradicts itself.

4. **Analyze the Fourth Statement**:
- The statement claims that "Ovules are not enclosed by ovary wall in gymnosperms."
- **Evaluation**: This statement is correct. In gymnosperms, ovules are exposed and not enclosed by an ovary wall, which is a characteristic feature of this group.

### Conclusion:
The correct statement is: **"Ovules are not enclosed by ovary wall in gymnosperms."**