Metals are usually hard. But exception to this fact is:

Options

A. zinc
B. copper
C. iron
D. sodium

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for an exception to the general property of metals being hard. We need to identify a metal that is not hard.

2. **Identifying Metal Groups**: Metals can be categorized into different groups. One important group to consider is the alkali metals, which include lithium, sodium, potassium, rubidium, cesium, and francium.

3. **Properties of Alkali Metals**: Alkali metals are known for having a single electron in their outermost shell. This characteristic leads to weaker metallic bonding compared to other metals.

4. **Weak Metallic Bonding**: The weak metallic bonding in alkali metals results in their softness. Because of this weak bonding, alkali metals can be easily cut with a knife.

5. **Examples of Soft Metals**: Among the alkali metals, sodium (Na) and potassium (K) are commonly referenced as examples of soft metals. They can be easily manipulated due to their softness.

6. **Conclusion**: Based on the properties discussed, the exception to the hardness of metals is sodium, which is an alkali metal and can be cut with a knife.

### Final Answer:
The exception to the fact that metals are usually hard is sodium (Na), which is an alkali metal.

---