Final answer:
The sentence with correct capitalization is 'Robbie doesn't know if he should send his package through UPS or the USPS,' as both UPS and USPS are acronyms that should be fully capitalized.
Step-by-step explanation:
The correct sentence that has proper capitalization is D. Robbie doesn't know if he should send his package through UPS or the USPS. In this sentence, both UPS and USPS are acronyms for United Parcel Service and United States Postal Service, respectively, and therefore should be capitalized entirely. It is important to use proper capitalization, especially for official names and acronyms, to convey the correct meaning and maintain a professional tone.