An asomycetous fungus is

Options

A. Yeast
B. Phytophthora
C. Puccinia
D. Agaricus

Answer

To determine which organism is an ascomycetous fungus among the given options, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Classification**:
- Ascomycetes, also known as sac fungi, are a group of fungi characterized by the formation of ascospores in a sac-like structure called an ascus.

2. **Evaluate the Options**:
- The options provided are:
- Yeast
- Phytophthora
- Paxinia
- Agaricus

3. **Analyze Each Option**:
- **Agaricus**:
- This is a genus of mushrooms, which can be edible or poisonous. It belongs to the class Agaricomycetes, not Ascomycetes.
- **Conclusion**: Not an ascomycetous fungus.

- **Phytophthora**:
- This is a genus of plant-damaging oomycetes, which are not true fungi but are often grouped with them.
- **Conclusion**: Not an ascomycetous fungus.

- **Paxinia**:
- This is also a genus of mushrooms and falls under the class Agaricomycetes.
- **Conclusion**: Not an ascomycetous fungus.

- **Yeast**:
- Yeast is a type of fungus that is crucial in fermentation processes. It is classified under Ascomycetes, also known as sac fungi due to the presence of ascospores.
- **Conclusion**: This is an ascomycetous fungus.

4. **Final Answer**:
- Based on the analysis, the correct answer is **Yeast**.