The design of the neck muscles and the spinal bones of the molluscs allow that it can pull in the exposed parts of the body such that the predator doesn’t find anything but a hard shell to bite.
A. allow that it can pull in the exposed parts of the body such that
Subject verb agreement error; "The design" is a singular subject but "allow" is a plural verb
B. allow it to pull in its exposed parts, and so
Subject verb agreement error; "The design" is a singular subject but "allow" is a plural verb
C. allows that it can draw in its exposed parts, and so
Correct
D. allows for it to draw in its exposed part, and that
Illogical meaning;
The design allows
that it can draw in... and
that predator doesn't find anything...
E. allows it to draw in its exposed parts, so that
so that implies a purpose however what we need is cause and effect, mollusk draw in its body parts and as a consequence predators can't bite it. This option implies that the design is deliberately trying that predator find nothing but a hard shell...Design is not a smart existence that can deliberately has a purpose for something but rather the designer sculpts the design for the intended purpose

Enough of philosophy for today