Final answer:
A ground moraine is an irregular thin till layer laid down by a retreating glacier, which is the correct answer to the student's question.
Step-by-step explanation:
The correct answer to the student's question is b) Ground moraine. A ground moraine is an irregular, usually thin till layer laid down by a retreating glacier. Ground moraines are deposits of glacial debris (till) that were carried at the base of the glacier and left behind as the glacier melts and retreats. This type of moraine is typically much less defined than end moraines, such as terminal or lateral moraines, which are left at the snout or sides of glaciers, respectively.
The student's question is related to the processes that occur when a glacier retreats. For instance, a retreating glacier can leave behind bare rock, where previously there was ice cover, visible when lava flows harden into rock or after events like landslides uncover areas of bare rock. Lateral and terminal moraines are accumulations of debris deposited along the sides and at the terminus of the glacier, respectively, which are much more substantial and easily distinguishable compared to ground moraines.