Electric wires in household appliances are typically covered with a plastic or rubber coating to ensure safety and proper functioning of the electrical system. Let's analyze the reason behind this covering and the provided options:

1. To make the wires heavier: Covering wires with plastic or rubber is not primarily intended to make them heavier. The weight of the wires does not contribute to their efficiency or safety in typical household applications.
2. To prevent electric shocks by insulation: This is the correct answer. The plastic or rubber coating acts as an insulating layer. Insulators are materials that do not conduct electricity well, thus preventing the flow of electric current to unintended areas, such as human contact, which could lead to electric shocks. This safety feature safeguards users while handling or operating electrical appliances.
3. To produce more heat: The purpose of the plastic or rubber coating is not to generate heat. In fact, these materials are selected specifically because they resist electrical flow, thereby not contributing to heat generation.
4. To increase the flow of current: Insulators do not enhance the flow of current. Instead, they limit electricity flow to the conductive core of the wire, ensuring that current travels through the intended circuit without dispersing.

Hence, the primary reason for the plastic or rubber coating on electric wires in household appliances is indeed to prevent electric shocks by insulation. This helps in ensuring the safety of the electrical system and its users.