Which service should a startup use to track and forecast monthly spending on Azure services?

Azure Cost Management is the correct choice because it provides tools to monitor, analyze, and optimize Azure spending. It allows users to track current costs, forecast future expenditures, and set budgets to control spending, which is crucial for a startup. Azure Lighthouse is for managing Azure resources across multiple tenants. Azure Monitor collects and analyzes telemetry data from Azure resources but doesn't primarily focus on cost tracking and forecasting. Azure Policy helps enforce organizational standards and assess compliance, not manage costs directly.