To find the marginal product of the additional worker, we need to calculate the increase in total production after hiring that worker.
Initially, with 5 workers producing 20 loaves each:
\[ 5 \text{ workers} \times 20 \text{ loaves/worker} = 100 \text{ loaves} \]
After hiring the additional worker, the total production is 125 loaves.
Now, we can find the marginal product by subtracting the initial production from the new production:
\[ \text{Marginal Product} = \text{New Production} - \text{Old Production} = 125 \text{ loaves} - 100 \text{ loaves} = 25 \text{ loaves} \]
Therefore, the marginal product of the additional worker is 25 loaves of bread.
The correct response is:
25 loaves of bread.