Final answer:
To improve the model performance on test data, I would recommend increasing the number of trees in the boosted tree model.
Step-by-step explanation:
To improve the model performance on test data, I would recommend trying option B, which is to increase the number of trees in the boosted tree model. Increasing the number of trees can help to improve the generalization ability of the model and reduce overfitting, which may be the reason for the lower AUC value on the test data. By increasing the number of trees, the model can capture more complex patterns in the data and potentially improve its performance on unseen data.