It will be 4) 31.2 10 to the 10th power.
Because 31.2 times 10 to the 10th power because you are multiplying by the 10s power you simply move the decimal to the left or the right.
Because the power is positive (+) you move to the right
If it is negative (-) you move to the left
The answer would be 31,200,000,000
Hope this helped!
:)