The sentence is divided into four segments: (A), (B), (C), and (D). Identify…

Concept

Parallelism (parallel structure) requires that two or more items joined in a series or by a coordinating conjunction such as and share the same grammatical form. When a verb like enjoys governs a list of activities, every item in that list must take the same form — most commonly the gerund (the -ing form), so that the list reads as a single consistent set.

Application

1. The main verb is enjoys, which introduces a list of activities the subject performs.

2. First item: hiking in the mountains — a gerund phrase (-ing form).

3. Second item: swimming in the lake — also a gerund phrase, matching the first.

4. Third item: to explore ancient ruins — an infinitive phrase. This switches form mid-list and breaks the parallel pattern set by the two gerunds.

5. The fix: change the infinitive to the gerund — exploring ancient ruins — so all three items match.

Cross-check

Rewriting the corrected sentence confirms the repair: She enjoys hiking in the mountains, swimming in the lake, and exploring ancient ruins during her travels abroad. Now all three list items are gerunds, the parallelism holds, and the sentence reads smoothly. The closing phrase during her travels abroad is an adverbial modifier, not a list item, so it does not need to match the gerund form.

Result

The error lies in the segment and to explore ancient ruins, because the infinitive to explore violates the parallel structure required by the list.