The algal forms are mostly marine in:

Options

A. Chlorophyceae
B. Phaeophyceae
C. Ascomycetes
D. All of these

Answer

To answer the question about which algal forms are mostly marine, we can analyze the options provided:

1. **Chlorophyceae (Green Algae)**:
- These algae are primarily found in freshwater environments. While some species can be found in marine habitats, they are not predominantly marine.

2. **Phaeophyceae (Brown Algae)**:
- This group of algae is mostly marine. They are typically found in coastal regions and are known for their large size and multicellular forms. Common examples include Sargassum and kelp, which thrive in marine environments.

3. **Ascomycetes**:
- This is not an algal group but a class of fungi. Ascomycetes do not fall under the category of algae and are not primarily marine.

4. **All of these**:
- Since not all the options provided are marine, this option cannot be correct.

Based on this analysis, the correct answer is **Phaeophyceae (Brown Algae)**, as they are the algal forms that are mostly marine.

### Step-by-Step Solution:
1. Identify the groups of algae mentioned in the options.
2. Analyze the habitat preferences of each group:
- Chlorophyceae: Mostly freshwater.
- Phaeophyceae: Mostly marine.
- Ascomycetes: Fungal group, not algae.
3. Determine which group is predominantly marine.
4. Conclude that the correct answer is Phaeophyceae.