Parallelism refers to the fact that the same form of a word is used throughout a particular sentence. Here, as you can see, the option D differs from the other options because the verb is used in its progressive form, instead of simple past form as in the previous options (were running, as opposed to dropped, got off, and yelled). This is why the correct option is D. were running towards the playground.
Which part of the sentence is not parallel with its other parts? A dropped their backpacks B got off of the school bus C yelled "hello" to their parents D were running towards the playground