'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.
I believe your answer would be D. Procure means to obtain something. So... viewing is the opposite of obtaining.
I think it is C.Traveling abroad
