The picture is a cross section through a coal seam, sandwiched between coal bearing rock below and shale and shaly sandstone above. A tongue of igneous rock, marked P, has forced its way into this pile of layers and spread out right along one bedding surface rather than punching straight through the layers. Naming this feature is really about knowing the vocabulary geologists use for intrusions based on their orientation to the surrounding beds.
dyke: a dyke is an intrusion that crosses the bedding at a steep angle, cutting through several layers like a wall. The body in the figure instead spreads sideways along a single contact, so it does not fit a dyke.
washout: a washout is formed when a stream channel eroded into the coal seam before burial, later filled in with sand. It is a sedimentary scour feature, not molten rock, so it does not match the igneous body shown in the figure.
sill: a sill is an intrusion that forces its way between existing layers and spreads out parallel to the bedding, following the path of least resistance along a weak layer boundary. This is exactly the geometry drawn in the figure, where P runs along the coal to sandstone contact.
seam split: a seam split happens when a coal seam divides laterally into two benches separated by a wedge of rock, caused by changes in the swamp that formed the coal. It involves no molten rock at all, so it cannot be the answer here.
Since the intrusion in the drawing spreads out along the bedding plane rather than crossing it, it fits the definition of a sill precisely, which makes option (C) correct.
Let's summarize:
An intrusion parallel to bedding is a sill; one that cuts across bedding is a dyke.
Washout and seam split are sedimentary features unrelated to igneous activity.
So the igneous body marked P, running along the bedding plane of the coal seam, is a sill.
