An electric transformer is an electric device that consists of only two coils wound on a common core. Let us break down the answer and explain it in detail:

Definition of an Electric Transformer:
An electric transformer is a static device that transfers electrical energy between two or more circuits through electromagnetic induction. It is commonly used to increase or decrease the voltage levels of alternating current (AC) electricity.

Components of an Electric Transformer:
The essential components of an electric transformer include:

1. Coils: The transformer consists of two coils, known as the primary coil and the secondary coil. These coils are made of insulated copper wire, and they are wound on a common core.

2. Core: The core is usually made of laminated iron or steel. Its purpose is to provide a closed magnetic circuit and reduce energy losses due to eddy currents and hysteresis.

Working Principle of an Electric Transformer:
When an alternating current passes through the primary coil, it creates an alternating magnetic field around the coil. This changing magnetic field then induces a voltage in the secondary coil through electromagnetic induction. The ratio of the number of turns in the primary coil to the number of turns in the secondary coil determines the voltage transformation ratio.

Explanation of the Correct Answer - Option D:
The correct answer is option D because an electric transformer consists of only two coils wound on a common core. This means that there are only two coils in the transformer, one primary coil and one secondary coil. These coils are wound on a common core, which provides a closed magnetic circuit for efficient energy transfer.

The other options (A, B, and C) are incorrect:

- Option A states that an electric transformer consists of two or more multi-turn coils wound on a common core. This is not true because a transformer typically has only two coils, not more.

- Option B states that an electric transformer consists of only two coils wound on a common core. This is incorrect because it does not mention the requirement of multi-turn coils.

- Option C states that an electric transformer consists of two or more multi-turn coils wound on a common core. This is incorrect because it suggests that a transformer can have more than two coils, which is not the case in a typical transformer.

In conclusion, an electric transformer is an electric device that consists of only two coils wound on a common core. This configuration allows for efficient energy transfer through electromagnetic induction.