The correct answer is A. Adult Kemp's ridley turtles are only two feet long.
Step-by-step explanation
The text talks about the Kemp's Ridley Sea Turtles, their size, the place where they usually make their homes in the Gulf of Mexico, the way the female lays her eggs in the sand on the beach, how the baby turtles are born, their size, how they reach the sea and their growth. According to the above, the sentence that best supports the idea that Kemp's Ridley Sea Turtles are one of the smallest species is "Adult Kemp's ridley turtles are only two feet long" because in that fragment of the text the author mentions their size after having mentioned the size of other species, making a comparison of sizes between several species. So the correct answer is A.