The feature marked X in the diagram is a "stack". A stack is a geological landform consisting of a steep and often vertical column or columns of rock in the sea near a coast. It forms when erosion processes such as wave action erode the softer, more easily weathered rock layers surrounding a harder rock formation, leaving only the harder material behind in the form of a column. Stacks are typically located a short distance from the coast and are often surrounded by water.
