Which of the following is a unicellular organism?

Options

A. Insect
B. Bird
C. Plant
D. Bacteria

Explanation

**Step-by-Step Solution:**

1. **Understand the Definition of Unicellular Organisms:**
- A unicellular organism is defined as an organism that consists of a single cell. This means that all the life processes of the organism are carried out by that one cell.

2. **Differentiate Between Unicellular and Multicellular Organisms:**
- Multicellular organisms are made up of many cells, which can be specialized for different functions. Examples include insects, birds, and plants.

3. **Identify the Types of Organisms:**
- Unicellular organisms are typically prokaryotes, which lack a well-defined nucleus and organelles. In contrast, multicellular organisms are eukaryotes, which have a well-defined nucleus and various cell organelles.

4. **Analyze the Given Options:**
- Review the options provided in the question. If the options include organisms like insects, birds, plants, and bacteria, we need to classify each:
- Insect: Multicellular
- Bird: Multicellular
- Plant: Multicellular
- Bacteria: Unicellular

5. **Conclude the Answer:**
- Since bacteria is the only option that is unicellular, it is the correct answer to the question.

**Final Answer:**
- The unicellular organism among the options is **bacteria**.

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