Pen : Ink :: Pencil : ?

The answer is C. Lead.

A pen is a writing instrument that uses ink to produce a mark on a surface. A pencil is a writing instrument that uses graphite to produce a mark on a surface. Graphite is a form of lead, so the answer to the question “Pen : Ink :: Pencil : ?” is C. Lead.

Option A, Knife, is incorrect because a knife is not used to write. Option B, Write, is incorrect because write is an action, not a substance. Option D, Chalk, is incorrect because chalk is a type of limestone, not a type of lead.