What is data leakage?

Question: Which situation is an example of data leakage? A) Scaling the entire dataset before splitting into train and test sets B) Using only training data to fit preprocessing steps C) Evaluating on a held-out test set D) Using cross-validation after a proper pipeline split Correct: A Explanation: Data leakage happens when information from the validation or test set influences training. Scaling the entire dataset before splitting allows statistics from the test set to affect the model pipeline, causing overly optimistic results. Topic: advanced ML / data leakage / validation