The correct answer is - glacial erosion and sea level rise.
The glaciers make U shaped valleys as they move downwards. When they come to the sea they melt (if the sea is not frozen as well). So once the whole glacier starts to melt it leaves behind a U shaped valley, and when the last ice age finished they did melt, but also the sea level rose, so the sea filled the U shaped valleys left by the glaciers, thus the fjords formed.