Which of the following substance are blue?

Options

A. `Fe(BO_(2))_(2)`
B. `CoAl_(2)O_(4)`
C. `Co(BO_(2))_(2)`
D. `NaCoPO_(4)`

Answer

To determine which of the given substances are blue, we will analyze each compound one by one based on their known colors.

### Step-by-Step Solution:

1. **Identify the First Compound: FeBO2 (whole twice)**
- **Analysis**: This compound is known to have a color that is typically green or yellowish-brown.
- **Conclusion**: **Not Blue**

2. **Identify the Second Compound: CoAl2O4**
- **Analysis**: This compound is commonly known as Cobalt Blue (or "Thénard's Blue"). It is well-known for its blue color.
- **Conclusion**: **Blue**

3. **Identify the Third Compound: CoBO2 (whole twice)**
- **Analysis**: This compound also contains cobalt and is known to exhibit a blue color.
- **Conclusion**: **Blue**

4. **Identify the Fourth Compound: NaCoPO4**
- **Analysis**: This compound is also known to be blue in color.
- **Conclusion**: **Blue**

### Final Summary:
- The substances that are blue among the given options are:
- CoAl2O4 (Cobalt Blue)
- CoBO2
- NaCoPO4

Thus, the correct choices are the second, third, and fourth compounds.