For example - string is 101 (n=3) then possible sub-strings are : (epsilon, 1,0,10,101,01,1) , so max length sub-string will be string itself.
And for accepting 'n' length string atleast 'n+1' states will be required ,either in DFA or NDFA (may require more states for acceptance of other sub-strings too)
So Answer is C) n+1