Which of the following is a standard unit of measurement?

• Handspan is the maximum distance between the tip of the thumb and the tip of the little finger of a fully stretched hand.
• Cubit is an ancient unit measurement of length where one cubit is the distance between the elbow and the tip of the middle finger.
• Pace is the distance covered in one normal step while walking.
• Handspan, cubit and pace cannot be used as standard units for the measurement of length because their size can vary from person to person.
• Whereas an inch is a standard unit of length where one inch is equal to 2.54 cm and its value does not change.

Therefore, 'Inch' is the correct response.