| Options | Replacement |
|---|---|
| cactus | Correct, as "Cactus" is the singular form of "Cacti". |
| cats | Incorrect, "Cats" bears no relation to "Cacti". |
| cactuses | Incorrect, "Cactuses" is an alternative plural form, not singular. |
| cactusas | Incorrect, "Cactusas" is not a recognized word. |