Which type of irregular rock formations develop within existing fractures and fissures in the Earth's crust?

Explanation

Sills are sheet-like igneous intrusions that exploit pre-existing cracks or bedding planes in the surrounding rock, resulting in irregular rock masses. Dikes also intrude cracks but are typically more vertical and tabular. Laccoliths form dome-shaped intrusions, while batholiths are large, deep-seated igneous bodies. Therefore, sills best fit the description of irregular rock masses formed in pre-existing cracks.