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

Let's check the alphabetical tracking order of letters for each option:
* Believe: $\text{B}(2)\to \text{E}(5)\to \text{L}(12)\to \text{I}(9)\dots$ (Incorrect, $9<12$)
* Cleanse: $\text{C}(3)\to \text{L}(12)\to \text{E}(5)\dots$ (Incorrect, $5<12$)
* Beefy: $\text{B}(2)\to \text{E}(5)\to \text{E}(5)\to \text{F}(6)\to \text{Y}(25)$. Here, each consecutive letter satisfies ${\text{rank}}_i\le {\text{rank}}_{i+1}$, which matches perfect alphabetical progression.