Which Azure tool helps a startup track and reduce cloud spending and assign budgets to individual departments?

Azure Cost Management + Billing is the correct tool because it provides robust features for tracking cloud spending, analyzing costs, creating budgets, and identifying areas for cost optimization. It allows startups to gain visibility into their Azure consumption and allocate costs to specific departments or projects. Azure Monitor focuses on collecting and analyzing telemetry data from Azure resources, not cost management. Azure Advisor provides recommendations for optimizing performance, security, reliability, operational excellence, and cost, but it's not the primary tool for detailed cost tracking and budgeting. Azure Lighthouse enables multi-tenant management for service providers, which is unrelated to internal cost management for a single organization.