When ordering decimals you want to look at the largest place value of each number. The largest place value will be the number farthest to the left. In this case, the ones place in all the numbers is the same, 2, so you would need to go to the next largest place value, the tenths place. Since 2.1 and 2.1021 have the same number in the tenths place, you will need to see which of them is larger by looking at the other place values. Since 2.1 has no other numbers to the right then this number would be smaller than 2.1021. So, 2.1021 is the largest and then 2.1 would be the second largest.
Next you will need to see out of the remaining two numbers, 2.007 and 2.07, which is the smallest. Since 2.007 has a zero in the next place value to the right (the hundredths), and 2.07 has a 7 in the hundredths, then 2.07 is greater than 2.007. So, the order from least to greatest would be 2.007, 2.07, 2.1, and then 2.1021.