Which is the best example of parallel structure in the following sentence? the sound of the booming fireworks bounced off the mountains and echo in everyone's ears. the sound of the booming fireworks bounce off the mountains and echo in everyone's ears. the sound of the booming fireworks bouncing off the mountain echoes in everyone's ears. the sound of the booming fireworks bounced off the mountains and echoed in everyone's ears.

Basically, to have parallel structure, a sentence should have the same basic parts of speech in the same tense.  Let's look at an example.  The following sentence does not have parallel structure:  I like to eat, running, and to sleep.  The following sentence does have parallel structure:  I like eating, running, and sleeping.  Thus, you'll note that the "ing" suffix for all elements of the list make this sentence parallel.  When we look at your sentences, we'll need to look at the verbs.  The first sentence's verbs are bounced/echo, which are not parallel because "bounced" is in the past tense, and "echo" is in the present tense.  With that in mind, we can see that the third sentence is the sentence that has parallel structure because the verbs, "bounced" and "echoed" are both in the past tense.