Which of the following nails is the longest?

The 20d nail is the longest among the options provided. The letter "d" stands for "penny," which is a historical term used in the U.S. to denote the length of nails. The number preceding the "d" indicates the nail's size, with higher numbers indicating longer nails.

Therefore, a 20d nail, being the largest numerical designation, will have a greater length than the other nails listed, such as the 8d, 16d, and 10d. For reference, a 20d nail typically measures around 3.5 inches in length, while the other sizes correspond to shorter lengths. Understanding this sizing system is essential in park maintenance and construction work, where using the correct nail size can affect the durability and stability of structures.