Although talking while gesturing with one’s hands was once considered undesirable, researchers have now found that TO USE GETURES WHEN YOU CONVERSE MAKES IT EASIER FOR MANY PEOPLE TO THINK AND SPEAK.
C) For the ease of many people during conversations one should gesture to think and speak.
D)Using gestures during conversation makes thinking and speaking easier for many people.
I was able to eliminate other choices. I don’t understand why C works because it agrees with the pronoun “one”?
Thanks

Choice C has “one” in it but is otherwise gobbledygook. It’s fine not to have another “one” in the second part of the sentence, as long as there’s not a “you” or something else that actively disagrees with the earlier “one.”