The correct answer is A. Seamus liked doing almost any physical activity: hiking, surfing, biking, running, and even spelunking.
Step-by-step explanation:
In grammar, a parallel structure in one in which there is a repetition of a grammatical structure, this usually implies a list of elements are presented using the same structure and related to the rest of the sentence in the same way which creates a parallel relation between all the elements presented.
In the case of presented, a parallel structure implies all the elements of the list which are sport or activities are presented by using the same grammatical structure which can be done by presenting all the elements using gerunds (verbs ended in -ing) or an infinitive (preposition "to" followed by the verb). Considering this, the only option which follows a parallel structure is "Seamus liked doing almost any physical activity: hiking, surfing, biking, running, and even spelunking" because all the elements after the colon (:) are presented using gerund grammatical structure which creates a parallel relationship between all the elements.