Explanation
Correct answer: B.
Choice B is the best answer. The convention being tested is subject-verb agreement. The plural verb "are" agrees in number with the plural compound subject "the trefoil knot and the figure-eight knot." The two types of knots joined by "and" function as a plural subject and thus require a plural verb.
Why the other choices are wrong
Choice A
Choice A is incorrect because the singular verb "was" doesn’t agree in number with the plural compound subject "the trefoil knot and the figure-eight knot."
Choice C
Choice C is incorrect because the singular verb "has been" doesn’t agree in number with the plural compound subject "the trefoil knot and the figure-eight knot."
Choice D
Choice D is incorrect because the singular verb "is" doesn’t agree in number with the plural compound subject "the trefoil knot and the figure-eight knot."