The stars, some of them at tremendous speeds, are in motion just as the planets are, yet being so far away from Earth that their apparent positions in the sky do not change enough for their movement to be observed during a single human lifetime.
"Them" in Choice B is not ambiguous because it appears in the modifying phrase "some of them at tremendous speeds". This phrase is modifying the clause in front of it, so logically, them could only refer to stars.
"They" is not ambiguous because pronouns in a new clause refer to the subject of the previous clause. In "B," the first clause is "Like the planets, the stars are in motion..." "Like the planets" is a modifying phrase, the independent clause is "the stars are in motion," and the subject of this independent clause is "the stars". So, the pronoun "they" in the new independent clause must refer to "stars."
"Them" in Choice B is not ambiguous because it appears in the modifying phrase "some of them at tremendous speeds". This phrase is modifying the clause in front of it, so logically, them could only refer to stars.
"They" is not ambiguous because pronouns in a new clause refer to the subject of the previous clause. In "B," the first clause is "Like the planets, the stars are in motion..." "Like the planets" is a modifying phrase, the independent clause is "the stars are in motion," and the subject of this independent clause is "the stars". So, the pronoun "they" in the new independent clause must refer to "stars."