The outer covering of which organism is used as abrasive for metal polishing ?
(a) Dinoflagellates
(b) Radiolarians
(c) Sponges
(d) Diatoms

Options

A. Dinoflagellates
B. Radiolarians
C. Sponge
D. Diatoms

Answer

To solve the question regarding which organism's outer covering is used as an abrasive for metal polishing, we will analyze the outer coverings of each of the organisms listed in the options.

### Step-by-Step Solution:

1. **Identify the Organisms and Their Outer Coverings:**
- **Dinoflagellates:** Their outer covering is made of cellulose, referred to as theca.
- **Radiolarians:** Their outer covering is composed of silica, but it is primarily used for oil exploration and geological dating.
- **Sponges:** They do not have a well-defined cell wall or membrane. Instead, they are covered by cells called pinnacocytes, which are not used as abrasives.
- **Diatoms:** Their outer covering is also made of silica, which has various commercial applications.

2. **Evaluate Each Option:**
- **(a) Dinoflagellates:** The cellulose outer covering (theca) is not used for metal polishing.
- **(b) Radiolarians:** Although they have a silica outer covering, it is not used for metal polishing but for oil exploration.
- **(c) Sponges:** They lack a defined outer covering suitable for abrasive use.
- **(d) Diatoms:** Their silica outer covering is known for its commercial uses, including being used as an abrasive for metal polishing.

3. **Conclusion:**
After evaluating all the options, the organism whose outer covering is used as an abrasive for metal polishing is **(d) Diatoms**.

### Final Answer:
**(d) Diatoms**