The correct answer is D. a.
The word “a” is used before singular nouns that are not specific. In this case, the speaker is not referring to a particular smartphone, but rather to smartphones in general. Therefore, the word “a” is the correct choice.
The word “the” is used before singular nouns that are specific. For example, if the speaker was referring to a particular smartphone that they had seen in a store, they would say “He does not know how to use the smartphone.”
The word “an” is used before singular nouns that begin with a vowel sound. For example, if the speaker was referring to a smartphone that had a blue screen, they would say “He does not know how to use an Android smartphone.”
The word “some” is used before plural nouns. For example, if the speaker was referring to a group of smartphones, they would say “He does not know how to use some smartphones.”