To find the percentage represented by the shaded squares in the grid model, you can use the following formula:
\[ \text{Percentage} = \left( \frac{\text{Number of shaded squares}}{\text{Total number of squares}} \right) \times 100 \]
In this case, there are 33 shaded squares out of a total of 100 squares:
\[ \text{Percentage} = \left( \frac{33}{100} \right) \times 100 = 33% \]
Therefore, the correct answer is 33%.