By comparing the place values, we can say that,
0.6426 is the smallest among the following as there is 0 in its units place.
Next comes, 6.426, as there is no digit in its tens place.
Between 64.26 and 64.026, the 64.026 is lesser than 64.26 as it has 0 in its tenths place and 64.26 has 2 in its tenths place.
So, the ascending order will be
0.6426 < 6.426 < 64.26 < 64.026