Option B - 'An' is used before a word that starts with a vowel sound. As 'white' starts with a consonant sound, we can't use 'an' here. Thus option B is incorrect.
Option C - 'The' is used before the definite subjects. As we do not know which white dog the subject is talking about, we can't use 'the' here. Thus option C is incorrect.
Option D - We use no article before the nouns that are usually uncountable. As 'white dog' is countable, we need an article. Thus option D is incorrect.
Option A - 'A' can be used here as 'white dog' is a singular subject and starts with a consonant sound. Thus option A is the correct answer.
'I have a black and a white dog.'