Which sentence uses parallelism? by working, playing, and dreaming together, we can build a better neighborhood. there is strength in numbers and working together. our major problems include worn-out roads, missing signs, and throwing trash everywhere. to clean up our open areas and having neighborhood potlucks would be a good start?

Parallelism means that the same form of a word is used throughout a particular sentence. Having this in mind, the correct option is the first sentence - By working, playing, and dreaming together, we can build a better neighborhood. 
You can see in this sentence that the same form of the verbs is used - all of them use the gerund form (verb + ing): working, playing, dreaming.
The other options are not so consistent.