To find the total cost of 100 bottles of juice at $1.39 each, Justin can recognize that multiplying by 100 is the same as moving the decimal point in $1.39 two places to the right (since 100 is \(10^2\)).
Therefore, the correct choice is:
He knows that 100 is 10 to the power of 2, so he needs to move the decimal point in 1.39 two places to the right.