A parallel structure refers to a type of grammatical structure in which elements of the same importance are presented using the same grammatical form or pattern in a sentence. In the sentence stem presented "He was sitting, reading, and..." there is a repetition "-ing" or gerund forms and therefore parallelism can be created if the last element includes this grammatical pattern. This means, adding planning, considering or eating would create parallelism, but adding a "thought about the book" does not create parallelism because this breaks with the repetition of the same grammatical structure that is necessary for parallelism.
