Which of the following uses a penny system to specify size?

The penny system, also known as the "dime" or "pennyweight" system, is a method used primarily in the construction and hardware industries to denote the size and length of nails. In this system, the term "penny" is traditionally written with a "d" suffix (for example, "8d" refers to an eight-penny nail) and originally originated from the price of a hundred nails in old English money.

Fishing nails specifically are a type of nail used in fishing or anchoring tasks, and they utilize the penny system to denote their size. This makes it easy for users to identify the right size of fishing nail for their specific requirements based on its length, which is extremely helpful in achieving optimal performance in fishing-related applications.

In contrast, bolts, screws, and washers typically use different sizing systems based on their dimensions, threads, and intended use rather than the penny system, which is specifically associated with nails. This distinguishes fishing nails as the correct answer in this context, making it essential to understand the sizing conventions applicable to different types of fasteners.