What is the pronoun of mud?

Mud doesn't have a pronoun. Pronouns are words that replace nouns, and mud is a noun.

To refer to mud, you would use a pronoun like:

* it (e.g., "The mud is very sticky.")

* this (e.g., "This mud is from the river.")

* that (e.g., "That mud is drying in the sun.")

It's important to remember that pronouns are used to replace nouns, and mud is a noun, not a pronoun.