The correct answer is D. no article.
The word “hockey” is a common noun, which means it refers to a general class of things. In this case, it refers to the sport of hockey. Common nouns are not typically preceded by articles.
The article “a” is used before singular common nouns that refer to something specific or unique. For example, “I saw a dog.” In this case, the speaker is referring to a specific dog, not just any dog.
The article “an” is used before singular common nouns that begin with a vowel sound. For example, “I saw an eagle.” In this case, the speaker is referring to a specific eagle, not just any eagle.
The article “the” is used before singular common nouns that refer to a specific thing that has already been mentioned or is known to the listener. For example, “I saw the dog that bit me.” In this case, the speaker is referring to a specific dog that has already been mentioned.
In the sentence “The only sport we enjoy is hockey,” the word “hockey” is not preceded by an article. This is because the speaker is referring to the sport of hockey in general, not a specific game or event.