Answer:
Joe correctly prepared the solution.
Explanation:
When comparing both methods, we see that the mass of potassium phosphate weighed is the same for Jennifer and Joe, so that's not a factor.
The difference lies in the fact that Jennifer added the solid to 1.0 liters of water, so the final volume is higher than 1.0 L, that's why the concentration will be lower than 1.0 M.
The final volume of Joe's solution is 1.0 L, that's why he prepared the solution correctly.