The correct answer is D. no article.
The word “day” is a common noun, which means it refers to any day in general. In this case, the speaker is not referring to a specific day, so they do not need to use an article.
If the speaker were referring to a specific day, they would use an article. For example, if they were talking about the day they got married, they would say “We stayed only the one day at the hotel.”
A is incorrect because “a” is used to refer to a specific day.
B is incorrect because “an” is used to refer to a day that begins with a vowel sound.
C is incorrect because “the” is used to refer to a specific day that has already been mentioned.