It's a standalone word with its own meaning, referring to something that supports or holds something else up.
Here's why it's not any of those:
* Prefixes come *before* a word to change its meaning (e.g., "pre" in "pre-game").
* Suffixes come *after* a word to change its meaning (e.g., "-ing" in "walking").
* Roots are the basic building blocks of words, often with their own meaning (e.g., "port" in "transport").
Let me know if you'd like to know about prefixes, suffixes, or roots in more detail!