Which one of the following cellular parts is correctly described

Options

A. Ribosomes - those on chloroplasts are larger (80s) while those in the cytoplasm are smaller (70s)
B. Lysosomes - optimally active at a pH of about 8.5
C. Thylakoids - flattened membranous sacs forming the grana of chloroplasts
D. Centrioles sites for active RNA synthesis

Answer

To determine which cellular part is correctly described, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Ribosomes**: The statement claims that ribosomes on the chloroplast are 80S, while those in the cytoplasm are 70S.
- **Analysis**: This statement is incorrect. Ribosomes in chloroplasts are actually 70S, while those in the cytoplasm of eukaryotic cells are 80S. Therefore, this description is not correct.

2. **Lysosomes**: The statement suggests that lysosomes are optimally active at a pH of about 8.5.
- **Analysis**: This statement is also incorrect. Lysosomes contain digestive enzymes that function best at an acidic pH, typically around 4 to 5. Thus, this description is wrong.

3. **Thylakoids**: The statement describes thylakoids as sac-like flattened structures forming the grana of chloroplasts.
- **Analysis**: This statement is correct. Thylakoids are indeed flattened membranous sacs that are part of the chloroplasts and are involved in the photosynthetic process. Therefore, this description is accurate.

4. **Centrioles**: The statement claims that centrioles are the site for active RNA synthesis.
- **Analysis**: This statement is incorrect. Centrioles are primarily involved in cell division, specifically in the formation of spindle fibers. They do not play a role in RNA synthesis.

### Conclusion:
Based on the analysis, the only correctly described cellular part is **thylakoids**, which are flattened membranous sacs forming the grana of chloroplasts.

### Final Answer:
**Thylakoids are correctly described as flattened membranous sacs forming the grana of chloroplasts.**

---