The correct answer is: C. 7 days
Formwork is a temporary structure used to support concrete during the curing process. It is typically made of wood, metal, or plastic, and is removed once the concrete has gained enough strength to support itself.
The time it takes for concrete to gain enough strength to support itself varies depending on a number of factors, including the type of concrete, the temperature, and the humidity. In general, however, concrete should be allowed to cure for at least 7 days before the formwork is removed.
Removing the formwork too early can result in the concrete cracking or breaking. This is because the concrete is not yet strong enough to support its own weight. On the other hand, leaving the formwork in place for too long can also cause problems. This is because the concrete can start to bond to the formwork, making it difficult to remove.
For walls, columns and vertical faces of all structural members, the form work is generally removed after 7 days. This is because the concrete has gained enough strength to support itself by this time. However, it is important to check the specific instructions for the type of concrete you are using, as the curing time may vary.