Answer: Option A
$$\eqalign{
& \,\,\,\,\,\,\,\,8|\overline {68} \,\,\overline {06} \,\,\overline {21} \,(824 \cr
& \,\,\,\,\,\,\,\,\,\,\,\,|\,\,64 \cr
& \,\,\,\,\,\,\,\,\,\,\,\,| - - - - - - - - \cr
& \,\,\,162|\,\,\,\,4\,06 \cr
& \,\,\,\,\,\,\,\,\,\,\,\,|\,\,\,\,\,3\,24 \cr
& \,\,\,\,\,\,\,\,\,\,\,\,| - - - - - - - - \cr
& 1644|\,\,\,\,\,\,\,\,\,\,82\,21 \cr
& \,\,\,\,\,\,\,\,\,\,\,\,|\,\,\,\,\,\,\,\,\,\,65\,76 \cr
& \,\,\,\,\,\,\,\,\,\,\,\,| - - - - - - \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,16\,45 \cr
& \therefore {\text{Number to be added}} \cr
& = {\left( {825} \right)^2} - 680621 \cr
& = 680625 - 680621 \cr
& = 4 \cr} $$