X represents a vertical intrusion of magma into the surrounding rock structure hence it must be a dike.

In contrast, a sill involves magma that flows between strata of sedimentary rock forming a thin but extensive sheet. A laccolith is a dome of igneous rock formed from a large enough volume of magma to raise up the overlying strata. A pluton is a body of intrusive igneous rock from solidified magma which has become exposed at the surface after years of uplift and erosion of the surrounding rocks.

The diagram below shows the formation of a pluton over time.