Lock out : When the counter enters one of the unused states,then it will not come back from that states ,and counter is said to be locked out.
a) MOD-3 : With 2FF we can have 4 states (0,1,2,3) , but here mod-3 implies 3 states say 0,1,2 if counter enters state 3 we cannot assure it will come back.
b) MOD-7 : One unused state , same explanation as above.
c) MOD-1: One unused state .
d) MOD-16 : No unused states ,
LOCK OUT not possible in MOD-16.