An example of an organically formed sedimentary rock is

An example of an organically formed sedimentary rock is coal.

Coal is formed from the remains of dead plants that accumulated in ancient swamps and wetlands. Over millions of years, these plant materials were buried under layers of sediment. As more layers built up on top, the increased pressure and heat transformed the plant remains into coal. The process involved is known as biochemical sedimentation, where organic materials are compacted and altered to form rock.

This is in contrast to the other options:

• Granite is an igneous rock, not sedimentary, formed from cooled magma or lava.
• Rock salt is a chemical sedimentary rock formed when water evaporates, leaving behind salt crystals.
• Sandstone is a clastic sedimentary rock formed from compacted sand particles.

To summarize, coal is the organically formed sedimentary rock among these options because it originates from ancient plant material that has undergone compaction and chemical transformation over time.