**Explanation:**

The correct answer is option 'D' - Fangs.

**Fangs:**
- Venomous snakes have specialized teeth called fangs that are used to inject venom into their prey.
- Fangs are long, hollow, and grooved teeth located in the upper jaw of the snake.
- They are connected to venom glands, which produce and store the venom.
- When the snake bites its prey, the muscles around the venom glands contract, forcing the venom through the grooves in the fangs.
- The venom is then injected into the prey's body, causing damage and often leading to paralysis or death.

**Tongue:**
- While the snake's tongue plays an important role in sensing its environment, it is not used to inject venom into prey.
- Snakes use their forked tongues to pick up scent particles from the air and ground.
- The tongue collects these particles and transfers them to a specialized organ in the roof of the snake's mouth called the Jacobson's organ.
- The Jacobson's organ helps the snake detect and interpret scents, aiding in finding prey and avoiding predators.

**Tail:**
- The tail of a snake does not play a role in injecting venom into prey.
- The tail is primarily used for balance and locomotion.
- Some snakes may use their tails as a defensive mechanism by rattling or vibrating them to warn potential threats, but they do not inject venom through their tails.

**Skin:**
- The skin of a snake is not involved in injecting venom into prey.
- However, the skin is an important organ for snakes as it provides protection, helps with thermoregulation, and allows for movement.
- It also allows snakes to shed their old skin periodically as they grow.

In summary, the fangs of a poisonous snake are the specialized teeth that inject venom into their prey. The tongue, tail, and skin do not play a role in venom injection.