The compound shown is 2,3-dibromobutane drawn without stereochemistry. Which of the four stereo drawings shown as options is its meso (achiral) stereoisomer?

2,3-Dibromobutane has two equivalent stereocentres, so its stereoisomers are $(2R,3R)$, $(2S,3S)$ and the meso $(2R,3S)$ form. Structure P is $(2R,3R)$ and Q is $(2S,3S)$; these are the enantiomeric pair. Structure S is 2,3-dibromopentane, a different compound with no meso form. Structure R has one $(R)$ and one $(S)$ centre on identical halves, so it has an internal mirror plane and is the meso isomer.