The correct answer is Azure Blob Storage with a Cool access tier. This is because Azure Blob Storage is designed to handle large amounts of unstructured data, like what is generated by IoT devices. The Cool access tier is optimized for data that is infrequently accessed and stored for at least 30 days, making it a cost-effective solution for this scenario. The Hot access tier is more expensive and meant for data that is accessed frequently. Azure Files would not be the best solution for large IoT datasets, as it is optimized for file shares for cloud or on-premises deployments. Azure Queue Storage is intended for storing large numbers of messages that can be accessed from anywhere in the world.