The correct answer is: C. Common variables are not used to apply transforms.
Common variables are variables that are present in both the training and test sets. They are used to create a missingness vector, which indicates which values in the test set are missing. They are also used to cut up quantitative variables, which means dividing them into smaller groups. However, they are not used to apply transforms.
A transform is a mathematical operation that is applied to a variable. For example, a common transform is the log transform, which is used to make the distribution of a variable more normal. Common variables are not used to apply transforms because they are not always relevant to the task at hand. For example, if you are trying to predict whether a customer will churn, it is not necessary to apply a transform to the customer’s age.
In conclusion, common variables are used to create a missingness vector, to cut up quantitative variables, but not to apply transforms.