What geological feature forms at the summit of a volcano and may hold water or lava?

The geological feature that forms at the summit of a volcano and may hold water or lava is a caldera. A caldera is a large depression resulting from the collapse of a volcano following an explosive eruption. When the magma chamber beneath the volcano empties, the ground above it can sink, creating this large, basin-like structure.

Calderas can be quite extensive and, over time, may collect water to form lakes. In some cases, they can also fill with lava from subsequent eruptions, depending on the volcanic activity in the area. This distinguishing characteristic of a caldera—being a significant depression at the summit of a volcano—sets it apart from other geological features.

Other options provide different volcanic structures: cones refer to the typical volcanic mountain shape formed by the accumulation of lava and ash, craters are usually smaller depressions formed directly at the vent where lava erupts, and fissures are cracks in the ground where lava can flow out but do not typically hold water or lava in a depression. Thus, the caldera is the most accurate answer in the context of volcanic summits.