100 is already rounded to one significant figure.
When expressing a number to one significant figure, you round it to the nearest power of 10. In this case, 150.932 to 1 significant figure would be 200. Since 150.932 is closer to 200 than to 100, the rounded value is 200.
It then is 40000 to one significant figure
It is 10
100
There is just one significant figure.