The correct answer is D. When you meet someone, you should say hello, smile, and shake hands.
Step-by-step explanation:
In grammar, a parallel structure or construction refers to the repetition of one grammatical pattern in a sentence. This commonly implies two or more elements that belong to the same category or are part of a list are introduced using the same grammatical structure. Considering this, the sentence that uses parallel structure is "When you meet someone, you should say hello, smile, and shake hands" because in this sentence the elements that belong to the same category "say hello, smile and shake hands" have the same grammatical structure which is the use of the base form of verbs or the verb without any modification, this means there is a repetition of a grammatical form in the three elements and therefore a parallel structure.