Final answer:
In K-fold cross-validation, the validation set is split into K subsets and the validation process is repeated K times.
Step-by-step explanation:
The correct statement about K in K-fold cross-validation is option c. K should always be a prime number. In K-fold cross-validation, the validation set is split into K subsets and the validation process is repeated K times. The purpose of K-fold cross-validation is to divide the dataset into K subsets to evaluate the performance of a machine learning model.