The correct option is C Both A and R are true and R is the correct explanation of A.
Standard unit of length is a fixed length all over the world, which does not vary with time, place, and persons and is not affected by any natural phenomenon.
Cubit is the length from the elbow to the fingertips. A cubit cannot be used as a standard unit of length as it is not fixed in length and varies from person to person.
Therefore, both A and R are true and R is the correct explanation of A.