Which of the following languages is not regular?

GATE · 2007 · CS· Modified — slightly modified from the official paper; see the solution

1. A.

   {wwR|w ∈ {0,1}+} 

2. B.

   {wwRx|x,w∈{0,1}+}

3. C.

   {wxwR|x,w∈{0,1}+}

4. D.

   {xwwR|x,w∈{0,1}+}

Attempted by 309 students.