Answer:
D. The mermaids swam in the sea, then combed their hair on the rocks.
Step-by-step explanation:
Example D is the only one which is correct. On example A we find the expression (the'ir) which is completely misspelled and nonexistent in English language. On the other hand, example B shows a misuse of the adverb "there" representing an homophone to the word "their", and finally, example C contains the contracted form of they are which also turns out to be an homphone to the word "their".