The correct answer is B. 4.
A fish plate is a metal plate that is used to connect two pieces of wood or metal together. It is typically used in construction and engineering applications. Fish plates are typically held in place by fish bolts, which are long bolts that are threaded on both ends. The fish bolts are passed through the fish plate and the pieces of wood or metal that are being connected, and then the nuts are tightened to secure the fish plate in place.
The number of fish bolts that are required to secure a fish plate depends on the size of the fish plate and the thickness of the pieces of wood or metal that are being connected. In general, four fish bolts are required to secure a fish plate. However, if the fish plate is large or the pieces of wood or metal are thick, more than four fish bolts may be required.
Option A is incorrect because two fish bolts are not typically enough to secure a fish plate. Option C is incorrect because five fish bolts are not typically used to secure a fish plate. Option D is incorrect because six fish bolts are not typically used to secure a fish plate.