This is Term Reference question. We can usually find the answer for this by looking at right before + after what is indicated.
“rate” is indicating how much serotonin is converted from tryptophan with intake of carbs, looking at the sentence JUST before the term, and also the very last sentence of the second paragraph. So, since it's not always right before + after that we should only look for, we should have the general idea of each paragraph.
Now, let's go to the answer choice.
A) This is exactly what we've talked about above. (Correct)
B) The author doesn’t specify how much carb has been taken in the second paragraph.
C) How much insulin secretion occurs is affected by the proportion of carbs (19), according to the passage.
D) Just indicated from the passage that depending on carbs' intake, insulin secretion increases.
E) Also affected by the proportion of carbs’ intake, and the author just said amount of tryptophan that enters into bloodstream increases with the amount carb induces insulin to secrete.