Which of the following is an example of class imbalance in machine learning?

A$.$ A training dataset for predicting a class label that has disproportionate representation of the available classes

B$.$ A prediction dataset for predicting a class label that has disproportionate representation of the available classes

C$.$ A categorical variable with multiple classes of data, where one or more of the classes was not expected within the dataset

D$.$ An independent categorical variable with unequal representation of the available classes or categories