all flip flops are clocked with same clock so their delay is max(5,4,3)=5micro sec
similarly all the nad gate are producing output after getting their input from flip flops so their max delay is also 5 micro sec (due to flip flops are synchronously arranged) so total delay =5+5 micro sec=10 micro sec
frequency =1/10micro sec
=0.1*10^6
=100000
=100Khz