How to find the minimum number of states required by a DFA
You can prove that your set is maximal in (at least) two different ways:


Show that every string is equivalent to one of the strings in your set.
Construct a DFA for the language having the same number of states as are strings in your set.


In your case both approaches are not too difficult.

Note, however, that the question only asks you to show that every DFA for the language must contain at least $k+2$ states. For this there is no need to show that your collection is maximal. If you find $k+2$ pairwise inequivalent strings, then it follows that every DFA for the language must contain at least $k+2$ states. If the collection is not maximal, all it means is that your bound isn't tight.