The governing principle is that a reamer only finishes a hole to the shape and size ground into its own cutting body; the geometry of the flutes and lands is what transfers to the workpiece.

A reamer is fed into an already-drilled hole to bring it to an exact diameter and a smooth surface, removing only a small amount of stock.

A straight reamer has cutting edges parallel to its axis, so it produces or finishes a hole of constant diameter along its length.

A taper reamer is ground so its body and cutting edges follow a defined taper angle; as it advances it progressively enlarges the hole into the matching tapered bore, for example for a taper pin.

Therefore the true distinguishing feature is that the taper reamer's edges are formed to match the required taper, directly shaping the tapered hole.

Handle length is unrelated to cutting action, so it cannot be the defining difference.

Flute count and serrated teeth affect chip clearance or roughing behaviour, not whether the hole comes out straight or tapered, so those options miss the real distinction.

Hence, the answer is A taper reamer's edges match the required taper.