Apart from amembranous nature, reasons for association of ribosomal sub-units are

Options

A. `Mg^+ concentration
B. demand for protein synthesis
C. both 1 and 2
D. Lack of DNA

Answer

### Step-by-Step Solution:

1. **Understanding Ribosomal Subunits**: Ribosomes are made up of two subunits, a large subunit and a small subunit, which come together during protein synthesis. They are not membrane-bound organelles.

2. **Identifying the Question**: The question asks for reasons for the association of ribosomal subunits apart from their membranous nature.

3. **Evaluating the Options**:
- **Option 1: Magnesium Concentration**: Magnesium ions are essential for the stability and association of ribosomal subunits. They help in maintaining the structural integrity of the ribosome, allowing the subunits to come together effectively. This option is correct.

- **Option 2: Demand of Protein Synthesis**: This option suggests that the need for protein synthesis drives the association of ribosomal subunits. However, the association of ribosomal subunits is a prerequisite for protein synthesis to occur, not a result of it. Therefore, this option is incorrect.

- **Option 3: Both First and Second**: Since Option 2 is incorrect, this option is also incorrect.

- **Option 4: Lack of DNA**: This option implies that the absence of DNA is a reason for the association of ribosomal subunits. However, ribosomal subunits do not require DNA for their association; they function independently of DNA. Thus, this option is also incorrect.

4. **Conclusion**: The only correct reason for the association of ribosomal subunits, apart from their membranous nature, is the presence of magnesium ions.

### Final Answer:
The correct answer is **Magnesium concentration**.