The structure of glycine at pH 6 is (isoelectric point = 6)

Question

$\begin{array}{1 1}(a)\;H_2N-CH_2COOH\\(b)\;H_2N-CH_2COO^-\\(c)\;H_3N^+-CH_2COOH\\(d)\;H_3N^+-CH_2COO^-\end{array} $