Which of the following is not a compound?

Options

A. Common salt
B. Water
C. Helium
D. Copper sulphate

Explanation

To determine which of the following is not a compound, we need to understand the definition of a compound and analyze the given options.

### Step-by-Step Solution:

1. **Understand the Definition of a Compound**:
- A compound is a substance formed when two or more different types of atoms bond together in a fixed ratio.

2. **Analyze the First Option - Common Salt (Sodium Chloride)**:
- Common salt has the chemical formula NaCl, which consists of sodium (Na) and chlorine (Cl) atoms.
- Since it contains more than one type of atom and they are chemically bonded, common salt is a compound.

3. **Analyze the Second Option - Water**:
- Water has the chemical formula H2O, which consists of two hydrogen (H) atoms and one oxygen (O) atom.
- It also contains more than one type of atom that are chemically bonded, so water is a compound.

4. **Analyze the Third Option - Helium**:
- Helium is represented by the symbol He and consists of only one type of atom (helium).
- Since it does not contain more than one type of atom, helium is not a compound; it is an element.

5. **Analyze the Fourth Option - Copper Sulfate**:
- Copper sulfate has the chemical formula CuSO4, which consists of copper (Cu), sulfur (S), and oxygen (O) atoms.
- This contains more than one type of atom that are chemically bonded, so copper sulfate is a compound.

6. **Conclusion**:
- Among the options analyzed, helium is the only substance that is not a compound. Therefore, the answer is helium.

### Final Answer:
Helium is not a compound.