Which of the following can be used as a standard unit of length?

• Handspan is the maximum distance between the tip of the thumb and the tip of the little finger of a fully stretched hand.
• The length of a forearm between the elbow and the tip of the middle finger is called a cubit.
• The length of the outstretched arms is called fathom.
• Handspan, cubit, and fathom are not standard units as their size can vary from person to person.
• Whereas a yard is a standard unit where one yard is equal to 3 feet or 0.9144 m.

Therefore, 'Yard' is the correct response.