Which of the following decimal numbers is the smallest?

1. 0.108

2. 1.08

3. 0.801

4. 0.81

Converting to like decimals (3 decimal places): 0.108, 1.080, 0.801, 0.810.

Clearly, 0.108 < 0.801 < 0.810 < 1.080. So, 0.108 is the smallest.

Hence, option 1 is the correct option.