The particular design of muscles and bones in the neck and limbs of the turtle allow
that it can draw in its exposed parts such that an attacker can find nothing
but hard shell to bite.

A. allow that it can draw in its exposed parts such that
B. allow it to draw in its exposed parts, and so
C. allows for it to draw in its exposed parts and that
D. allows that it can draw in its exposed parts, and so
E. allows it to draw in its exposed parts, so that

For thsi sentence you nee dteh singular form since it refers to "the particular design", so A and B are out.
C- Allows for it is idiomatically worng
D- "and so" is not the correct expression
E is the right answer since it uses the idiom "so that"

+1 for E.