Xavier
X-ray
xanadu
X-ray is already a proper noun.
The proper noun is Jessica, the name of a person. A proper noun always starts with a capital letter.
Nope. Common noun.
xenon
David's
NO!!! 'Janet' is a proper noun. It is a female forename. derived from the male 'John'. Being a proper noun it starts with a capital letter.
xaiver
The word 'World X' is a proper noun, the name of a company, World X Productions, LLC. A proper noun is the name of a person, place, thing, or a title. A proper noun is always capitalized. However, the word is a common noun when not referring to the above company. For example, 'When world X conflicts with world Y, a science fiction tale is born.' This use of 'world x' is a common noun.
proper
Proper nouns.