The phrase, hardened magma intrusion, best describes a sill. The correct option id B.
What is a sill?
Sill, also known as sheet, is a plain igneous rock intrusion that shapes among previously existing slabs of rock.
Sills form parallel to the bedding of the rocks that surround them, and while they can have vertical to horizontal orientations, flat sills are the most common.
A sill is a concordant intrusive sheet, which means it does not cut through existing rock beds. At high magma flux, sill stacking creates a sill complex and a large magma chamber.
Thus, the correct option is B.
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