Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is the use of plural nouns and possessive determiners. The plural noun "jugglers" and the plural possessive determiner "their" correctly indicate that there are multiple jugglers whose motion is being discussed.
Why the other choices are wrong
Choice A
Choice A is incorrect because the context requires the plural noun "jugglers" and the plural possessive determiner "their," not the plural possessive noun "jugglers’" and the contraction "they’re."
Choice B
Choice B is incorrect because the context requires the plural possessive determiner "their," not the word "there," which means "in that place."
Choice C
Choice C is incorrect because the context requires the plural noun "jugglers," not the singular possessive noun "juggler’s."