What is the purpose of cross-validation in model training?

The purpose of cross-validation in model training is to assess how the results of a statistical analysis will generalize to an independent dataset. It involves partitioning the original dataset into subsets, training the model on some subsets while validating it on the remaining subsets, and then repeating this process multiple times. This approach helps in evaluating the model's performance in a more reliable manner by ensuring that the model is not just fitting to the specific training data but is able to generalize well to unseen data.

This generalization capability is crucial because the ultimate goal of a predictive model is to perform well on new, unseen data rather than just performing well on the dataset used for training. By using cross-validation, data scientists can obtain a better estimate of the model's effectiveness when applied in real-world scenarios.

The other options, while relevant to model training and evaluation, do not capture the primary objective of cross-validation. For instance, testing the speed of model training is more related to performance benchmarks, optimizing hyperparameters focuses on fine-tuning model settings for better performance, and increasing the accuracy of the training data does not represent the process of validating model generalization. Thus, the second choice is the clearest representation of cross-validation's purpose.