Wood is the worst conductor of electricity among the listed options, making it a natural insulator. Electricity flows easily through materials called conductors, and poorly through materials known as insulators. Among the choices provided — wood, copper, iron, and water — wood is the least capable of conducting electricity.

Wood is a non-metallic and organic material. In its dry form, it contains no free electrons, which are necessary for the flow of electric current. That’s why dry wood is considered an insulator. It does not allow electricity to pass through easily, which is why it’s often used in tools or materials that need to prevent electric shock — like wooden handles on electrician’s tools.

Now let’s understand the other options:
- **Copper** is one of the best conductors of electricity and is widely used in electric wires and circuits.
- **Iron** also conducts electricity well, though not as effectively as copper, and is used in some electrical applications.
- **Water**, especially tap or natural water, can conduct electricity because it contains dissolved salts and minerals that allow current to flow. Pure water is a poor conductor, but pure water is rarely found in nature.

In contrast, dry wood contains no such ions or free electrons, so its extremely poor at conducting electricity. However, it’s important to note that **wet wood** can become a conductor because the water absorbed contains impurities.

So, the correct answer is **Wood** — it is the least conductor of electricity among the given materials and serves as a natural insulator in many safety applications.