From the following pairs determine the correctly described cellular parts.

Options

A. Thylakoids - Flattened membranous sacs forming the grana of chloroplast.
B. Centrioles - Sites for active RNA synthesis
C. Ribosomes - Those in chloroplasts are large (80 s) while those in the cytoplasm are smaller (70 s)
D. Lysosomes - Optically active at a pH of about 8.5

Answer

To determine the correctly described cellular parts from the given pairs, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Evaluate the first statement: Thylakoid - flattened membranous sac forming the grana of chloroplasts.**
- Thylakoids are indeed flattened, membranous sacs that are part of the chloroplasts. They stack to form structures called grana (singular: granum), which are essential for the photosynthetic process.
- **Conclusion:** This statement is correct.

2. **Evaluate the second statement: Centrioles are the site of RNA synthesis.**
- Centrioles are cylindrical structures involved in cell division and the formation of cilia and flagella. They do not play a role in RNA synthesis; that function is primarily associated with the nucleus and ribosomes.
- **Conclusion:** This statement is incorrect.

3. **Evaluate the third statement: Ribosomes in chloroplasts are large 80S while those in the cytoplasm are similar to 70S.**
- Ribosomes in chloroplasts are actually 70S, while ribosomes in the cytoplasm of eukaryotic cells are 80S. Therefore, this statement incorrectly describes the sizes of ribosomes in these locations.
- **Conclusion:** This statement is incorrect.

4. **Evaluate the fourth statement: Lysosomes are optically active at the pH of about 8.5.**
- Lysosomes are membrane-bound organelles that contain digestive enzymes and are active at a pH of around 4 to 5, which is acidic. A pH of 8.5 would not be conducive for lysosomal activity.
- **Conclusion:** This statement is incorrect.

### Final Conclusion:
- The only correct statement is the first one regarding thylakoids. Thus, the correct answer is option A.