Which of the following regular expression allows strings on {a,b}* with length n where n is a multiple of 4.
(a) (a+b+ab+ba+aa+bb+aba+bab+abab+baba)*
(b) (bbbb+aaaa)*
(c) ((a+b)(a+b)(a+b)(a+b))*
(d) None of the mentioned
This question was posed to me in a national level competition.
Query is from Regular Languages and D-PDA topic in section Push Down Automata of Automata Theory