Look at the pattern given:
If we consider a single figure, then what pattern does the number of squares in each row follow?
Odd numbers
In the pattern above, let's take the first figure.
It has one row and there is only 1 square in it.
In the second figure, it has 1 square in its first row and 3 squares in its second row.
Hence, in each figure, the number of squares in each row is an odd number. Also, the numbers are in increasing order starting from 1. Hence, the pattern followed by the number of squares in each row is that of the odd numbers.