Which of the following examples would be considered a natural monument?

A natural monument typically refers to a naturally occurring feature in the landscape that has significant geological or ecological importance. This can include elements such as unique rock formations, ancient trees, or other natural features that are notable for their beauty or significance in the environment. In this case, trees and rocks are prime examples of natural monuments as they represent unaltered natural elements that contribute to the biodiversity and geological diversity of an area.

The other options involve human-made objects or modifications to the landscape. Metal pins and concrete markers are artificial installations used for various purposes, often related to surveying or boundary marking. Roads and fences represent human modifications to the environment that serve practical functions like transportation or property delineation but do not reflect natural features of the landscape. Thus, trees and rocks are the fitting representation of natural monuments in this context.