To track the total number of trails associated with a park without writing code, the correct actions to take are:
Use a roll-up summary field on the Park record to show the total number of Trails (B). Roll-up summary fields calculate and display a value in a master record based on the values of fields in a detail record. They are used to count, sum, average, or get the minimum/maximum of values in related detail records.
Use a master-detail relationship between the Park and Trail objects (C). Master-detail relationships are necessary for roll-up summary fields as they allow the master object to control certain behaviors of the detail object and summarize data from those detail records.
A formula field (A) cannot dynamically count related records unless those records are linked via a master-detail relationship, and it cannot alone handle counts across related records without such a relationship. A lookup relationship (D) does not support roll-up summaries unless combined with additional tools like triggers or third-party apps, which involve coding or extra configuration outside standard object setup.
For more details, refer to the Salesforce Help documentation on master-detail relationships and roll-up summary fields:
Master-Detail Relationship: https://help.salesforce.com/articleView?id=relationships_considerations.htm&type=5 Roll-Up Summary Fields: https://help.salesforce.com/articleView?id=fields_about_roll_up_summary_fields.htm&type=5