If you have seen all the discussions before, you should find out that most of the people chose E. However, in my humble opinion,neither D or E is perfect.
D's mistake is "allows that," which means an action that doesn't match the idea of this context. E's problem is "so that," which doesn't carry the original logical meaning of the context. It gives people the wrong idea that the turtle's design permits the attackers not to find its head. Further more, there shouldn't be a coma in front of "so that." A better choice should be like the following: The particular design of muscles and bones in the neck and limbs of the turtle
allows it to draw in its exposed parts, and so attacker can find nothing | 
