Select the correct statement from the following.

Options

A. Activated sludge-sediment in settlement tanks of sewage treatment plant is a rich source of an aerobic bacteria.
B. Biogas is produced by the activity of aerobic bacteria on animal waste.
C. Methanobacterium is an aerobic bacterium found in the rumen of cattle.
D. Biogas, commonly called gobar gas, is pure methane.

Answer

To solve the question, we need to analyze each statement provided in the options and determine which one is correct.

### Step-by-Step Solution:

1. **Evaluate Option A**:
- The statement claims that "activity-resistant sediment in settlement tanks of sewage treatment plants is a resource of aerobic bacteria."
- In sewage treatment, the sediment that settles in the settling tank is known as activated sludge, which is indeed rich in aerobic bacteria. This statement is correct.

2. **Evaluate Option B**:
- This option states that "biogas is produced by the activity of aerobic bacteria on animal waste."
- Biogas is primarily produced by anaerobic bacteria that decompose organic matter in the absence of oxygen. Therefore, this statement is incorrect.

3. **Evaluate Option C**:
- The statement claims that "methanobacterium is an aerobic bacterium found in the rumen of cattle."
- Methanobacterium is actually an anaerobic bacterium that plays a role in the digestion process in the rumen. Thus, this statement is also incorrect.

4. **Evaluate Option D**:
- This option states that "biogas commonly called gobergas is pure methane."
- Biogas is a mixture of gases, primarily methane (50-70%) and carbon dioxide (30-40%), along with small amounts of other gases. Therefore, this statement is incorrect.

### Conclusion:
After evaluating all the options, we find that only Option A is correct.

**Final Answer**: The correct statement is Option A: "activity-resistant sediment in settlement tanks of sewage treatment plants is a resource of aerobic bacteria."