Starting from second line or row i.e. 1 1 , every line describes it's preceding line from left to right .

For example, second line says 1 1 which actually tells you that there's only one 1 in the preceding (first) line. Likewise, third line says 2 1 which denotes that there are two 1's in the preceding (second) line. Similarly, fourth line (one 2 one 1) describes third line ( 2 1 ), and fifth line (one 1 one 2 two 1) tells about fourth line ( 1 2 1 1). So, the mystery line should describe fifth line (1 1 1 2 2 1), which is just --------> three 1 two 2 one 1

3 1 2 2 1 1

is the correct answer.