The primary bromide shown reacts with aqueous $\mathrm{NaOH}$ by SN2. Which structure is the substitution product?

Substitution occurs at the CH2Br carbon, which is not the stereocentre. The stereocentre (C-2 of 1-bromo-2-methylbutane) is untouched, so its spatial arrangement is retained and the product is the corresponding 2-methylbutan-1-ol with the same geometry. The enantiomeric alcohol would require a bond to the stereocentre to break; the tertiary alcohol would need a rearranged cation; the alkene is an elimination product.