Formula fields in Salesforce can use various functions including text, mathematical, and logical functions to create simple or complex expressions. They can handle data from fields of the same record. Option C, "A combination of the Opportunity's Text and a Description fields," is a valid configuration for a formula field as it involves simple concatenation of text fields, which is supported.
Options A, B, and D exceed the capabilities of formula fields:
A: Hyperlinks to other objects using formula fields do not directly support navigation to parent records of a different object.
B: Rich Text and HTML formatting are not supported in formula fields.
D: Formula fields have limits in their complexity and field references which would make mixing numerous fields from different related objects impractical and likely exceed formula size limits.
Reference: Formula Field Overview on Salesforce Developer Guide