Causal organism of wheat rust is:

Options

A. Puccinia
B. Alternaria
C. Mucor
D. Ustilago

Explanation

**Step-by-Step Solution:**

1. **Identify the Question**: The question asks for the causal organism of wheat rust. Wheat rust is a disease that affects wheat plants.

2. **Understand Wheat Rust**: Wheat rust is also known as leaf rust. It is important to know that this disease is caused by a specific type of fungus.

3. **Review the Options**: The options provided are:
- A) Succinim
- B) Alternaria
- C) Mucor
- D) Acetyl aldehyde

4. **Research the Causal Organism**: The causal organism of wheat rust is *Puccinia triticina*. This fungus is an obligate parasite, meaning it can only survive on living host plants.

5. **Identify the Correct Answer**: Among the options provided, the correct answer is option A, which refers to *Puccinia*. The other options do not cause wheat rust:
- *Alternaria* is known for causing late blight in potatoes.
- *Mucor* is a type of mold that does not cause wheat rust.
- Acetyl aldehyde is a chemical compound and not a biological organism.

6. **Conclusion**: Therefore, the correct answer to the question "Causal organism of wheat rust is:" is *Puccinia triticina*.