Im pretty sure it would be Ariel.
Tamar reads 600 words in 5 minutes, so he is already out of the question since Levi reads 700 words in 5 minutes. (Levi reads 140 words in 1 minute, so add 140+560=700 because they read 560 in 4 minutes)
So I think it’s Ariel because in 4 minutes she reads 600, and where the 5 would be, if you go straight up, it’s over 700 (if i had to guess, 750 since it’s counting by 100s and it would be in the middle of 700 and 800)