The correct answer is - fjords.
During the last Ice Age, the glaciers had carved narrow long valleys between the rocky mountains on the western part of Norway with their movement, weight, and pressure, on their way towards the sea. Once the Ice Age came to an end, the glaciers melted, the sea levels went up, and the narrow valleys were filled with water from the sea, thus creating the narrow sea inlets with high steep sides, that nowadays we refer to as fjords.