Final answer:
Biological weathering of a rock is best described by tree roots cracking the rock, as living organisms contribute to the breakdown of rocks, unlike chemical and physical weathering which involve other processes.
Step-by-step explanation:
The option that best describes biological weathering of a rock is B) Tree roots cracking the rock. Biological weathering refers to the process by which living organisms, such as plants and animals, contribute to the breakdown of rocks and minerals. For instance, tree roots grow into cracks in rocks and as the roots increase in size, they can exert pressure on the rock causing it to fracture and break apart. This form of weathering is a key part of the rock cycle as it aids in the gradual disintegration of these materials, which in turns forms soil over time. Other factors, such as acid rain dissolving rock or water freezing and expanding inside the rock, represent chemical and physical weathering, respectively. It is important to note that these forms of weathering, while different in nature, can work concurrently to break down rocks.