(C) Association model — This recommendation ("customers who bought X also bought Y") identifies relationships/associations between products purchased together, which is exactly what an Association model does.
The Association model (a sub-category of Unsupervised Learning) finds interesting relationships between variables in a database — e.g., bread buyers also buy butter. The textbook explicitly states such associations are used to recommend items to customers. Classification predicts categories, Regression predicts continuous values, and Clustering groups similar data points — none of these match "co-purchase" pattern finding.