The correct answer is D.
The word “dog” is a common noun, which means it refers to any member of the species Canis lupus familiaris. In this case, we are not referring to a specific dog, so we do not need to use an article.
The article “a” is used before singular common nouns that refer to a general or unspecified thing. For example, “I saw a dog.”
The article “an” is used before singular common nouns that begin with a vowel sound. For example, “I saw an apple.”
The article “the” is used before singular common nouns that refer to a specific thing. For example, “I saw the dog that bit me.”
In this case, we are not referring to a specific dog, so we do not need to use an article.