To go third, a variable must have less than three letters that come in front of it. There also must be at least two variables that could go in front.
To solve these questions, look at the letter in the answer choice, and count. I recommend physically using your finger; it’s much harder to make a mistake, and it forces you to take action.
E is CORRECT. H, J and Q all have to go in front of P, so P can go fourth at the earliest.
For the other answers, I’m going to say who could go in front of the letter to make them third:
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