The amino acid shown has $p{K}_{a1}=2.21$ and $p{K}_{a2}=9.15$. Which of Structures A to D is the predominant form, with the same configuration at the stereocentre, in a solution of pH 6?

At pH 6, between 2.21 and 9.15, the carboxyl group is ionised and the amino group is protonated, so the zwitterion predominates. Converting groups to ions does not alter the spatial arrangement, so the drawing must keep the same stereo descriptor as the molecule shown (D-serine, R); the enantiomeric zwitterion is wrong, and the other two have net charge.