Define cross-validation rigorously and compare k-fold, stratified k-fold, leave-one-out, nested CV, and time-series rolling/blocked CV. For a dataset with temporal ordering and class imbalance, design an evaluation that avoids leakage while providing stable estimates; justify fold construction and metric choice. Explain bias–variance tradeoff quantitatively: how model complexity, training data size, and regularization shift bias and variance; how CV error curves reveal under/overfitting; and which levers (features, model class, regularization, data augmentation) you would pull when variance is high vs. when bias is high.