Well assonance does mean repeating vowels sounds like for example “seems, beam , green “ you would just have to find something with the same vowel sounds on your paragraph or book
I believe it is the third option
'Exert' means to apply force on influence over something else, this can be in a negative or positive way. In this quotation, it means that language, if used correctly, has the ability to express a 'hidden power'. It's also worth mentioning that the simile is relevant as the moon has an influence on the level of tides on Earth.