(A) doesn’t establish that gesturing helps people quickly find the phrases they want. It simply establishes that most people under ordinary conditions regularly use gestures. But it doesn’t create a causal relationship between gesturing and speaking more quickly / repeating one’s self less often.

The origin of the cartoons that were described has no impact on the reasoning. We want to establish that gesturing helps people find phrases more quickly. (B) doesn’t do that.

The premises describe an experiment in which gesturing was correlated with quicker speech and less repetition. According to (C), then, the correlation observed in the experiment would establish that gesturing helps speakers find the phrases they want quickly.

We want to prove that gesturing helps speakers quickly find the phrases they want. We don’t want an answer that tells us what happens IF gesturing helps speakers quickly find the phrases they want. In other words, we’re looking for “gesturing correlated with speaking quickly / less repeats → helps speakers find phrases quickly.” (E) puts “helps speakers find phrases quickly” on the left side of the arrow.

(E) might strengthen the argument by strengthening the correlation between gesturing and speaking faster / repeating less. But it doesn’t prove, with 100% certainty, a causal relationship.