Which suffix indicates one who studies or practices?

The suffix that indicates one who studies or practices is -ist.

Here are some examples:

* Biologist: Someone who studies biology.

* Artist: Someone who practices art.

* Pianist: Someone who plays the piano.

* Physicist: Someone who studies physics.

* Politicist: Someone who is involved in politics.

Note: While -ist is the most common suffix, there are other suffixes that can indicate one who studies or practices, such as -er, -or, and -ian.