Which phrase correctly follows the capitalization rules?

The phrase "English language" correctly follows the capitalization rules because "English" is a proper adjective derived from the proper noun "England," thus it is capitalized. In the context of language and national identities, proper adjectives are always capitalized to indicate their significance and origin. The term "language" itself does not require capitalization in this context, as it is a common noun.

The other phrases do not adhere to the standard capitalization rules. For instance, "professor Henry" should have "Professor" capitalized as it is a title used before a person's name. "History class" is not capitalized because "history" is a common noun in this context and does not refer to a specific course name. Lastly, "Christmas day" should be written as "Christmas Day," with both words capitalized since it refers to a specific holiday.