Answer:
A. 100 g of this solution contains 39 g of phosphoric acid
Explanation:
The formular for percentage by a mass id usually given as;
( Mass of element / Mass of compound ) * 100%
So when a solution contains 39% by mass of phosphoric acid, it means that in 100 units of the solution, there are 39 units of phosphoric acid.
The only option that satisfies this is option A. 100 g of this solution contains 39 g of phosphoric acid