Question

which is the best location for storing radioactive wastes?
in the sand of a remote island
in chambers carved into the rock of a mountain
in the lava tubes of an active volcano
in natural caverns formed by the action of a river

Explanation:

Storing radioactive waste requires a stable, isolated environment. Remote islands' sand may be affected by water erosion. Active volcanoes are geologically unstable. Natural caverns formed by rivers may be prone to flooding or water - related geological changes. Chambers carved into mountain rock offer a more stable, isolated, and less likely to be disturbed (by water or geological activity in the short - term) location.

Answer:

in chambers carved into the rock of a mountain