With no explanation, chose the best option from "A", "B", "C" or "D". was indeed barred (or limited) under the mandatory Guidelines regime, see Koon, 518 U.S. at 92-95, 116 S.Ct. 2035, the alternative sentencing methodology employed by his sentencing court did not treat the Guidelines as mandatory. Nor did the court exclude or limit its consideration of any category of sentencing factors. To the contrary, the court extended Simpson an open-ended invitation to make mitigating arguments, regardless of whether they would have been available under the Guidelines scheme. See Sentencing Hr’g Tr. 25-26. And Simpson responded to that invitation by citing a series of factors in support of a sente 440 (D.C.Cir.1995). In later cases, we settled on a plain error standard of review for such claims. See United States v. Draffin, 286 F.3d 606, 609 (D.C.Cir.2002) (<HOLDING>); In re Sealed Case, 204 F.3d 1170, 1173

A: holding that failure to identify ground of admissibility limited appellate review to plain error
B: holding that the district courts denial of a downward departure was an exercise of discretion and therefore not reviewable
C: holding that we may not review a district courts refusal to grant a downward departure unless the court mistakenly believed that it lacked the authority to grant such a departure
D: holding that the failure to grant a sentencing departure on an unrequested ground is reviewable only for plain error
D.