Final answer:
A landslide is the environment most likely to deposit breccia because it involves rapid accumulation without transport, creating angular fragments. A braided river is characterized by sand deposits with gravel and cross-bedding, while dunes show large-scale cross-bedding with well-rounded grains.
Step-by-step explanation:
The environment most likely to deposit a breccia is a landslide. Breccia is a sedimentary rock consisting of angular fragments that are larger than 2 millimeters in diameter. The angular shape of the fragments indicates a lack of transport and suggests rapid accumulation and deposition, as occurs during a landslide.
Now, describing the different environments based on the given descriptions, the environment described as 'Mostly sand deposits with some sediment beds having a layer of gravel on the bottom and cross-bedding' is a braided river. Braided rivers are characterized by multiple channels that interweave, depositing mainly sand and gravel, often with cross-bedding indicative of shifting and dynamic water flow.
On the other hand, the description of sandstone with cross-bedding and well-rounded sand grains more closely resembles a dune environment, where wind-blown sand is deposited. Such environments are known for their large-scale cross-bedding, a result of aeolian processes. This kind of sedimentary structure gives clues about the wind direction at the time the sediment was deposited.