The correct answer is A. 1 cm per metre length.
A precast concrete pile is a type of foundation that is made from concrete that has been cast in a mold and then transported to the site where it will be used. The taper of a precast concrete pile is the difference in the diameter of the pile at the top and the bottom. The taper is typically specified by the manufacturer of the pile and is designed to provide the necessary strength and stability for the pile.
The taper of a precast concrete pile should not be more than 1 cm per metre length. This is because a greater taper could result in the pile being weaker and less stable. Additionally, a greater taper could make it more difficult to install the pile.
The other options are incorrect because they are greater than the maximum recommended taper of 1 cm per metre length.