Which of the following was not found in free form during origin of life :

Options

A. Ammonia
B. Methane
C. Oxygen
D. Hydrogen

Answer

To answer the question "Which of the following was not found in free form during the origin of life?" we can follow these steps:

### Step 1: Understand the Context
The question is asking about the elements or compounds that were not available in their free or uncombined state during the early stages of life on Earth.

### Step 2: Identify the Options
The options provided are:
1. Ammonia
2. Methane
3. Oxygen
4. Hydrogen

### Step 3: Analyze the Primitive Atmosphere
During the origin of life, the Earth's atmosphere was primarily reducing. This means it had a lot of hydrogen and other gases like water vapor, carbon monoxide, and carbon dioxide.

### Step 4: Determine the Presence of Each Option
- **Hydrogen**: Present in abundance in the primitive atmosphere.
- **Ammonia (NH3)**: Formed from the combination of nitrogen and hydrogen; thus, it was present but not in free form.
- **Methane (CH4)**: Also formed from the combination of carbon and hydrogen; thus, it was present but not in free form.
- **Oxygen (O2)**: Did not occur in free form because it readily combined with other elements to form oxides (like iron oxides and silicon oxides).

### Step 5: Conclusion
Based on the analysis, the element that was not found in free form during the origin of life is **Oxygen**.

### Final Answer
The correct answer is **Oxygen** (Option 3).

---