The National air and space Museum is one of many free Museums in washington, D. C. Which sentence is capitalized correctly? The National Air and space Museum is one of many free Museums in Washington, D. C. The national Air and Space museum is one of many free museums in Washington, D. c. The National Air and Space museum is one of many free Museums in washington, D. C. The National Air and Space Museum is one of many free museums in Washington, D. C.