Fishes have great flexibility when they swim because they have
a. separate vertebrae
c. scales
b. no limbs
d. no skin

Let's think about what is important for flexibility, or what is the biggest obstacle for flexibility in humans: it's the bones. So having separate vertebrae (this is the correct answer) would be very good for flexibility, while no limbs would not matter as much.  (and answers c and d are equivalent: they describe the same property in fish)