Which of the following ribosomes are engaged in protein synthesis in animals cell:

Options

A. Ribosomes which occur on nuclear membrane and E.R
B. Ribosomes of only cytosol
C. Ribosomers of only nucleolus and cytosol
D. Ribosome of only mitochondria and cytosol

Answer

To determine which ribosomes are engaged in protein synthesis in animal cells, let's analyze the options provided:

1. **Understanding Ribosomes**: Ribosomes are the cellular machinery responsible for synthesizing proteins. They can be found in different locations within the cell, including the cytosol, endoplasmic reticulum (ER), and mitochondria.

2. **Analyzing the Options**:
- **Option A**: Ribosomes which occur on the nuclear membrane and ER (endoplasmic reticulum).
- The rough endoplasmic reticulum (RER) is studded with ribosomes and is directly involved in the synthesis of proteins that are either secreted from the cell or incorporated into the cell's membranes. Ribosomes on the nuclear membrane can also be involved in protein synthesis.
- **Option B**: Ribosomes of only cytosol.
- Ribosomes in the cytosol primarily synthesize proteins that function within the cytosol itself. While they are important, they do not encompass all protein synthesis in the cell.
- **Option C**: Ribosomes of only nucleolus and cytosol.
- The nucleolus is primarily involved in the synthesis of ribosomal RNA (rRNA) and assembly of ribosomes, not in protein synthesis directly. Thus, this option is not correct.
- **Option D**: Ribosomes of only mitochondria and cytosol.
- Mitochondria have their own ribosomes and synthesize some proteins, but they are limited to mitochondrial proteins. This option does not cover the full scope of protein synthesis in the cell.

3. **Conclusion**: The ribosomes on the nuclear membrane and the endoplasmic reticulum (option A) are crucial for synthesizing proteins that serve various functions in the cell, including those that are secreted or integrated into membranes. Therefore, the correct answer is **Option A**.

### Final Answer:
**Option A**: Ribosomes which occur on nuclear membrane and ER (endoplasmic reticulum) are engaged in protein synthesis in animal cells.