Prufrock thinks of the frolicking mermaids as creatures who—

(in “the love song of j. alfred prufrock”)
a) would fit well with his social set.
b) crawl on the bottom of the sea.
c) live in a world of freedom and beauty.
d) know that the ocean is safer than land

Here is a stanza from “The Love Song of J. Alfred Prufrock”,which refers to the frolicking mermaids.

I have seen them riding seaward on the waves 
Combing the white hair of the waves blown back 
When the wind blows the water white and black. 
We have lingered in the chambers of the sea 
By sea-girls wreathed with seaweed red and brown 
Till human voices wake us, and we drown.

After reading the above stanza,I have concluded the following observations:

Prufrock thinks of the frolicking mermaids as creatures who lure him toward death and destruction.
Also,He thinks of the frolicking mermaids as creatures who crawl on the bottom of the sea.
Hence,
The best answer is :
B) crawl on the bottom of the sea.