We want to get the angle complementary to angle BOC i.e the angle that when added to BOC gives 90 degrees.
We can see that the angles [tex]\begin{gathered} \angle AOB+\angle DOB=180 \\ \text{but }\angle AOB=90^{\circ} \\ so,\angle DOB=90^{\circ} \\ \text{but}\angle DOB=\angle BOC+\angle DOC \\ \text{Therefore, }\angle BOC+\angle DOC=90^{\circ} \end{gathered}[/tex]Therefore, the answer is
