To make a phosphate buffer at pH 12.38 starting with one liter of 10mM phosphoric acid (H3PO4; pKs are of 2.15, 6.82, and 12.38), you could add:
a) 15 millimoles of KOH
b) 20 millimoles of HCL
c) You cannot make a buffer by adding HCL or KOH
d) 25 millimoles of HCL
e) 25 millimoles of KOH