The correct answer is B. Intense exercise performed only once in a while.
Explanation:
A parallel structure refers to the repetition of a grammatical form or pattern in a sentence, this usually means a set of elements relate to another element in the same way in terms of grammar. In this way, all the elements in a sentence relate in the same way, which means they are parallel this shows all elements have the same level of importance. In the case of the sentence "Moderate exercise performed every day is more beneficial than " the option that would create a parallel structure is "Intense exercise performed only once in a while", this can be explained as the first part of the sentence "Moderate exercise" is composed by an adjective (Moderate) and a noun (exercise) and therefore, the second part in a parallel sentence should be composed in the same way "Intense" (adjective) and "exercise" (noun). Using "-ing" forms, beginning by to or any other options will stop the structure from being parallel as it is required a repetition of patterns or the same grammatical structure to create parallelism.
