A change of units does not change the number of significant figures in a measurement. The original value $2.308\text{ cm}$ has 4 significant figures. In $0.02308\text{ m}$, the leading zeros are not significant, keeping the significant figures at 4. The other options change the precision and have 5 significant figures (e.g., $23.080\text{ mm}$ and $0.000023080\text{ km}$ contain an extra trailing zero, which is significant).