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

Let's examine the alphabetical order of letters for each option:
* Collops: $\text{C}(3),\text{O}(15),\text{L}(12)\dots$ (No, $\text{O}>\text{L}$)
* Envelops: $\text{E}(5),\text{N}(14),\text{V}(22),\text{E}(5)\dots$ (No, $\text{V}>\text{E}$)
* Bloops: $\text{B}(2),\text{L}(12),\text{O}(15),\text{O}(15),\text{P}(16),\text{S}(19)$
Checking positions: $2\le 12\le 15\le 15\le 16\le 19$. This is strictly non-decreasing alphabetical order.
* Adrops: $\text{A}(1),\text{D}(4),\text{R}(18),\text{O}(15)\dots$ (No, $\text{R}>\text{O}$)

Hence, "Bloops" has its letters in alphabetical order.