Final answer:
The term for an assembly of witches is 'coven,' and by adding the letter 'h' you get 'cloven,' which describes a split hoof.
Step-by-step explanation:
The word meaning "an assembly of witches" is coven. If you add the letter 'h' to coven, you get the word cloven, which is a term describing a hoof that is split into two parts. Therefore, the correct answer is that the word describing a hoof you get when you add a letter to a word meaning "an assembly of witches" is cloven.