The question asks about the type of formwork used for continuous vertical structures like chimneys, silos, and towers. Let's evaluate the options:

1. Slip Formwork: Slip formwork is a method of concrete construction that is used for creating vertical structures rapidly. This formwork is a continuously moving form that is used for pouring concrete. It is particularly advantageous for tall structures like chimneys, silos, and towers, where traditional formwork techniques would be less efficient. As the formwork is moved vertically in a continuous process, it allows for a seamless construction process without joints.
2. Steel Formwork: Steel formwork is typically used for repetitive usage and for construction projects requiring high-quality finishes. While it is durable and provides a smooth finish, it is not specifically designed for continuous vertical structures like those mentioned in the question.
3. Plastic Formwork: Plastic formwork is lightweight and reusable, often employed in sites where cost-effectiveness is a priority. However, it is not ideally suited for very tall or continuous vertical structures like chimneys or towers, as it does not offer the necessary framework support for such heights.
4. Timber Formwork: Timber formwork is a traditional method used for many types of construction due to its flexibility in size and shape. It is typically used for smaller projects or when a single-use formwork is needed. It does not offer the same advantages as slip formwork for continuous vertical structures.

Based on the above analysis, the correct answer is Slip Formwork, as it is specifically designed for the continuous construction of tall vertical structures.

Conclusion: Slip formwork provides a seamless and efficient way to construct continuous vertical structures by moving as the concrete sets, ensuring a uniform structure without horizontal joints.