Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is the use of verb forms within a sentence. The plural verb “help” provides the relative clause beginning with “that” with a finite verb and agrees in number with the plural subject “coiled branching structures.”
Why the other choices are wrong
Choice A
Choice A is incorrect because the singular verb “has helped” doesn’t agree in number with the plural subject “coiled branching structures.”
Choice B
Choice B is incorrect because the singular verb “helps” doesn’t agree in number with the plural subject “coiled branching structures.”
Choice C
Choice C is incorrect because it results in an ungrammatical sentence. The nonfinite participle “helping” doesn’t supply a finite verb to the relative clause beginning with “that.”