To find the constant of proportionality, we need to calculate the ratio between the change in the number of gems collected and the change in the number of levels completed.
First, we calculate the change in the number of gems collected: 600 - 450 = 150.
Next, we calculate the change in the number of levels completed: 8 - 6 = 2.
The constant of proportionality is then given by the ratio of these two changes: 150/2 = 75.
Therefore, the correct answer is p = 75.