Final answer:
The most likely explanation for the presence of Upper Paleolithic artifacts in the lowest level of the cave is time displacement.
Step-by-step explanation:
The most likely explanation for the presence of Upper Paleolithic artifacts in the lowest level of the cave is time displacement. Time displacement refers to the phenomenon where artifacts from different time periods are found in the same location due to natural processes such as erosion or sedimentation. In this case, it is possible that the Upper Paleolithic artifacts were deposited in the lower level of the cave through natural geological processes.