The correct answer is option 'D' - "a".

Explanation:
To understand why option 'D' is the correct answer, let's first review the sentence structure and the rules for using articles (a, an, the).

The sentence "He does not know how to use _______ smartphone" is a statement about a specific smartphone that the person referred to as "he" does not know how to use. In English grammar, when we talk about a specific object, we use the definite article "the". However, in this sentence, the word "smartphone" is not referring to a specific smartphone, but rather to smartphones in general. Therefore, we need to use an indefinite article.

Indefinite articles (a, an) are used when we refer to a non-specific or generic object. In this case, "a" is the correct indefinite article to use because "smartphone" starts with a consonant sound.

Using option 'D' - "a" in the sentence, it becomes "He does not know how to use a smartphone." This sentence correctly conveys the idea that the person referred to as "he" does not know how to use any smartphone in general, without specifying a particular one.

In summary, the correct answer is option 'D' - "a" because the sentence is referring to smartphones in general, not a specific smartphone.