Which of the following organisms have a size of only `0.3 mu m` in
length?

Options

A. Mycoplasma
B. Euglenoids
C. Slime moulds
D. All of these

Answer

To determine which organism has a size of only 0.3 micrometers in length, we can analyze the options provided:

1. **Mycoplasma**:
- Mycoplasma are known to be among the smallest living organisms, with sizes ranging from approximately 0.2 to 0.3 micrometers. They are classified as bacteria and are known for lacking a cell wall. This makes them unique and allows them to survive in various environments, including those without oxygen.

2. **Euglenoids**:
- Euglenoids are a group of protists that typically range in size from about 15 to 500 micrometers. They are larger than mycoplasma and are characterized by their flagella and ability to photosynthesize.

3. **Slime Molds**:
- Slime molds are also members of the kingdom Protista and can vary significantly in size, often being much larger than mycoplasma. They can range from a few micrometers to several centimeters in size.

4. **All of these**:
- This option suggests that all the listed organisms share the same size of 0.3 micrometers, which is not accurate based on the previous analysis.

Based on this information, the only organism that fits the size criterion of 0.3 micrometers is **Mycoplasma**.

### Final Answer:
The organism that has a size of only 0.3 micrometers in length is **Mycoplasma**.