Columnar joints form when a. a lava flow erupts under deep water b. during the final stages of cooling, a lava flow contracts, forming tall polygonal fractures c. a lava flow encounters groundwater, which causes the flow to rapidly fracture d. a lava flow cools very rapidly, causing the flow to shatter in a chaotic pattern

b. during the final stages of cooling, a lava flow contracts, forming tall polygonal fractures

Explanation:

The columnar joints are very interesting and attractive for the eye geological features. They are not very common feature which makes them even more interesting. Some of them have been covered by sediments so they are hidden below the surface, but some have remained above the surface, sticking out as the erosion acts much slower on them than the surrounding area. They form when the lava cools off very quickly at its final stages, thus starting to contract, and instead of being smooth, they become polygonal fractures that are very tall.

it's a b and

snell's law

refractve index = sin incidence / sin refraction