Which of the following vibrators is used for internal concrete vibration?

The turbine vibrator is specifically designed for internal concrete vibration, making it the correct choice for this question. Internal vibrators, such as turbine vibrators, are inserted directly into the concrete mix to ensure proper compaction and removal of air bubbles. This process helps achieve a dense, uniform concrete structure and enhances its overall strength and durability.

Turbine vibrators operate by using a rotating mass that generates vibrations, effectively consolidating the concrete as it is poured. This method is crucial in preventing issues such as honeycombing or air pockets, which can weaken the concrete.

Other types of vibrators, such as hydraulic, pneumatic, and external vibrators, serve different functions and applications. For instance, hydraulic and pneumatic vibrators are often used for different materials or in scenarios that do not necessitate internal concrete consolidation. External vibrators are typically affixed to forms or structures and work on the surface rather than being inserted into the concrete itself. Therefore, they are not suitable for achieving the deep compaction needed within a concrete mix.