A Sequence

The next number describes the no. of digits in the previous number. 1 has one 1, hence 11 is the next number. 11 has two 1's so 21 is the next number, 21 has one 2 and one 1, so 1211 is the next number, and so on.

The last digit of all the terms in the sequence is 1. Hence the next term will also have last digit 1 and there is only 1 option with last digit 1. Thus, the answer is option D.

You can also try reading out the numbers. For example, read the first line - one-one. Write how it sounds in the next line. Read the subsequent line now - two-one. Write how it sounds in the third line. Read the third line now, one-two, one-one etc...

observe last two digit of number .. u will understand the answer easily …. it's like 11, 21, 11, 21, 11, 21, 11 …. so , in option i just select the number which has 21 in last two digit :)