The answer to your question would be that to create a parallel structure in this sentence, the phrase that would best replace the words "worked online" is the following one: Was working online. That is, the correct option would be C. Consequently, the sentence would be as follows: By the time she finished college, she knew she was happier being outdoors and observing the environment than when she was working online and interpreting statistics.
Parallel structure, also called parallelism, is the repetition of a chosen grammatical form within a sentence. By making each each compared item or idea in your sentence follow the same grammatical structure, you create a parallel construction.
