To determine the number of old-growth trees per acre, we can calculate the unit rate by dividing the total number of old-growth trees by the total area:
Total old-growth trees = 600
Total area (in acres) = 100
Number of old-growth trees per acre = Total old-growth trees / Total area
Number of old-growth trees per acre = 600 trees / 100 acres = 6 trees per acre
Therefore, the correct conclusion the biologist can make about the forest is:
The forest has 6 old-growth trees per acre.