Which of these are long, narrow inlets of the sea bordered by steep cliffs – most of them caused by glacial erosion?

The correct answer is fjord - a fjord is a long, narrow inlet of the sea bordered by steep cliffs - most of them caused by glacial erosion. Most of the fjords in the world can be seen in Scandinavia, where it's cold most of the time.
A plateau is like a valley, taiga is a type of a forest, and I'm not really sure what a timberline is, but I know it's not the correct answer.

Probably north korea if your family has decided to use only nuclear power.

Don’t use up to much of them it leads to pollution