What defines the fields that can be used in the Explore's UI?

The fields parameter defines which fields are available for users in the Explore's user interface. This parameter allows developers to specify particular dimensions and measures that can be accessed when users interact with the Explore. By using the fields parameter, you can control the visibility and accessibility of different aspects of your data model, ensuring that only relevant fields are available for exploration.

The other options pertain to different concepts within Looker. For instance, explore_fields is not a recognized parameter, explore_expose does not exist in this context, and view_connections typically refer to how views are linked within the LookML model rather than the accessibility of fields in the Explore UI. Hence, the fields parameter accurately captures the intended functionality of determining which fields can be utilized in the Explore interface.