Which of the following sources cannot be directly connected to Time Series Insights?

Time Series Insights is a service designed to help users explore and analyze time-series data at scale. It primarily facilitates connections to sources that have been specifically built or designed to provide time-stamped data for analysis.

IoT Hub allows you to connect and manage IoT devices, facilitating the ingestion of time-series data which can then flow into Time Series Insights. Event Hubs is another service that supports high-throughput real-time event streaming and works well for sending data into Time Series Insights for further analysis.

Cosmos DB, while it can store enormous amounts of data and is suitable for a variety of workloads, is not natively designed for direct integration with Time Series Insights. Unlike Event Hubs or IoT Hub that stream data in real-time and provide time-stamped events, Cosmos DB serves primarily as a database service where data may not inherently include the time series implications unless explicitly structured to do so. Therefore, direct connectivity from Cosmos DB to Time Series Insights is not supported, making it the correct answer in context of the question.