Absorption.
Thick absorption, like GIK 244 or Monstertraps. You can place Alpha series versions of the 244 and/or Monster but the benefits will be small at that close of a distance.
Do not use pure diffusers like the QRD, MLS type, etc... if your listening position is that close as comb filtering can occur.
Best,
Anand.
P.S. I used GIK products just for illustrative purposes. You can use the equivalent of 4 inch thick absorption or more from other manufacturers (RealTraps, Primacoustic, Vicoustic, etc...) as you see fit.