Given that this is the only information you have tk answer the question, you must assume that you don’t need any outside information to answer it, i.e. everything you need is in the prompt itself.
You should first rule out both “straightly” and “flexibly.” Neither of them work. I don’t think we even use “straightly” as an adverb with “flow.” We say “the river flows straight into the lake,” just as we say “the guards at the gate stood straight.”
Flexibly doesn’t work either. Things that “flow” aren’t flexible, because they don’t bend, break, or retain a solid form.
“Emotionally” could make sense, but when we look at “evenly” we see it is the better option of the two. Remember that the verb is “flow,” and we know it should “flow... easily.” For a speech to flow “easily” means that is delivered in a calm, natural, steady, measured way. “Evenly” most closely means the same thing, since an “even” flow is also calm, steady, and measured. “Emotional,” on the other hand, usually means agitated, dynamic, swelling and receding in turn, and so is the opposite of the meaning intended by “flow... easily.”