Which of the following is an example of unsupervised learning?

Unsupervised learning deals with unlabeled data and aims to find hidden patterns or structures within it. Grouping customers based on purchase histories (clustering) is a classic example, as the model identifies natural segments without prior knowledge of customer categories. Classifying images as dogs or cats is supervised learning because the model is trained on labeled images (e.g., "dog" or "cat"). Predicting a house price is also supervised learning, as it uses labeled data (house features and corresponding prices) to learn a mapping function. A model that learns to play chess by trial and error is an example of reinforcement learning, where an agent learns optimal actions through rewards and penalties in an environment.