The correct answer is C. 100 mm.
The compressive strength of cement is determined by testing a 100 mm cube of cement. The cube is placed in a compression testing machine and is subjected to a load until it fails. The compressive strength of the cement is then calculated as the load divided by the area of the cube.
The 100 mm cube is the most common size used for testing the compressive strength of cement. This size was chosen because it is a convenient size to handle and test. The 100 mm cube is also a representative size of the concrete that will be used in construction.
The other options are not correct because they are not the standard size for testing the compressive strength of cement. The 50 mm cube is too small and the 150 mm cube is too large. The 70.6 mm cube is not a standard size.