L1.L2 IS REGULAR L2.L1 IS ALSO REGULAR
if L1.L2 is regular then the automata will be like the start state will be from L1 and final will be from L2 so to design automata for L2L1 you just need to reverse the final and initial state ande the arrows so L2L1 is regular
if the languages are equal set difference will be empty
third statement $\sum ^{*}$ \ L is nothing but which are present in $\sum ^{*}$ but not in L that is the complement itself so all are correct no option is matching
