The correct answer would be the last one: That the monument of Colonel Shaw, like the aquarium, will one day likely be destroyed and forgotten.
"For the Union Dead" is a poem that was written by Robert Lowell as part of a bigger collection titled with the same name (and published in 1964), and which was first published in 1960. In this poem, what Lowell does is to reflect on the changes that are taking place in his home in Boston, due to advancements and social and technological development, but which are not necessarily the best, and are threatening a way of life that was accomplished thanks to the efforts of others in history. As such, he uses the image of Colonel Shaw, and his infantry of all-black soldiers, who went to the Civil War to fight against slavery and racial discrimination, and whose memory is preserved through a monument that is now being threatened with destruction by development, just as the old acquarium has already been destroyed. To put forth this idea, Lowell makes use in the beginning and end of the image of bursting bubbles, both regarding the fish he once watched as a child in the aquarium, and the bubble over which Colonel Shaw stood, which represented his desire to end racial discrimination and racism in general. In both instances, the bubbles burst, one because the aquarium has been torn down for improvements, and the other because racial discrimination is still rampant and the cause against it is being abandoned by people.
I've never read it but use a classic generic answer like "it has a great plot and the writing is very good. It teaches some important lessons that can be applied in life. Overall, many people could benefit from reading it."