Which of the following is an example of a nested function?: $*$

a$.=$COUNTIF$($A$2$:A$12,$"Risk"$)+$ SUMIFS$($B:B$,$C:C$,$$A$$1,$D:D$,$"Sequence"$)$

b$.=$IF$($A$2=$B$6,$COUNTIF$($C$2=$B$6,$"Monopoly"$),$"Chess"$)$

c$.=$COUNTA$($B:B$)$ & "Pictionary" & COUNTBLANK$($C:C$)$

d$.=$AVERAGE$($$B$$12$:$C$$19)$ & AVERAGEIFS$($$B$$12$:$C$$19,$$B:$$12$:$C$$19,$"Clue",$D:$$12$:$E$$19,$"Checkers"$)$