A nucleus is present in :

Question

Accepted Answer

Explanation:

1. **Option 2: Amoeba**  
   Amoeba is a type of single-celled organism that belongs to the group of eukaryotes. Eukaryotic cells are characterized by having a well-defined nucleus enclosed within a nuclear membrane. The nucleus contains the cell's genetic material and is essential for various cellular processes, including growth, metabolism, and reproduction. Amoebas, being eukaryotic, have a distinct nucleus, which makes Option 2 the correct answer.

2. **Option 1: Mycoplasma**  
   Mycoplasma is a genus of bacteria that lacks a cell wall and is classified as a prokaryote. Prokaryotic cells do not have a true nucleus. Instead, their genetic material is located in a region called the nucleoid, which is not enclosed by a membrane. Therefore, Mycoplasma does not have a nucleus, making Option 1 incorrect.

3. **Option 3: Bacteria**  
   Bacteria are also prokaryotic organisms, similar to Mycoplasma. They do not possess a true nucleus; instead, their DNA is found in the nucleoid region. The absence of a membrane-bound nucleus in bacteria means that Option 3 is incorrect.

4. **Option 4: Blue-green algae**  
   Blue-green algae, also known as cyanobacteria, are a group of photosynthetic bacteria. Like other bacteria, cyanobacteria are prokaryotic and do not have a true nucleus. Their genetic material is not enclosed within a nuclear membrane, which makes Option 4 incorrect.

In conclusion, the presence of a nucleus is a defining feature of eukaryotic cells, such as those of Amoeba, which is why Option 2 is the correct answer. The other options, being prokaryotic, do not have a true nucleus.