Which ArcGIS product exposes data as services?

ArcGIS Server is the product specifically designed for exposing data as services. It enables users to publish, manage, and share geographic information through various types of services, such as map services, feature services, and image services. By providing these services, ArcGIS Server allows users to consume and interact with geospatial data over the web, thus facilitating GIS applications and workflows.

In contrast, ArcGIS Desktop primarily serves as a client application for analysis and manipulation of geospatial data rather than focusing on publishing that data as services. ArcGIS Online functions as a cloud-based platform for sharing maps and applications, but it relies on services published by ArcGIS Server or through other means for data access. ArcGIS Collector is a mobile application designed for field data collection, allowing users to gather and update geographic information but not to serve or publish data as services. Therefore, the uniqueness of ArcGIS Server in its function as a service provider makes it the correct answer.