Final answer:
The description given corresponds to rivets, which are permanent mechanical fasteners that start out as thin shanks and are deformed to join parts together. Rivets fit pre-drilled holes and are often made of various metals.
Step-by-step explanation:
The question refers to a type of permanent mechanical fastener that begins as a shank slightly thinner than the hole it is meant to fill when joining two parts. This description fits most closely with rivets, which means the correct answer to the given question is A. Rivets.
Rivets are a type of fastener that are inserted into pre-drilled holes through the parts to be joined. They are then 'upset' or deformed on the end opposite to the head, creating a second head and clamping the parts securely together. Rivets are often made of metals like steel, aluminum, or copper, depending on the application, and do not require access to the back side of the assembly, unlike bolts or screws.
The mention of a metal peg in a hole in a metal block as a method of getting a tight fit refers to a common method of assembly in engineering, contrasting with the mortise and tenon joint used in woodworking. Similarly, a square peg fitting through the square hole and a round peg fitting through the round hole of a children's toy illustrates a metastable fit, not a permanent fastener like a rivet, Bolt, or screw.