Which of the following words has its letters in the exact English alphabetical order?

Let's check the alphabetical order of letters in each word:
* Acepots: A (1), C (3), E (5), P (16), O (15) $\to$ Broken order because $P>O$.
* Acerous: A (1), C (3), E (5), R (18), O (15) $\to$ Broken order because $R>O$.
* Acerty: A (1), C (3), E (5), R (18), T (20), Y (25) $\to$ Alphabetical indices are strictly increasing ($1<3<5<18<20<25$). All letters are in proper alphabetical order.
* Acephal: A (1), C (3), E (5), P (16), H (8) $\to$ Broken order because $P>H$.

Thus, Acerty is the correct answer.