Correct spelling


Correct spelling, explanation: protesters is made from the base word protest, and two suffixes: -er, which gives it the meaning of a person who protests, and the suffix -s, making it plural. Nothing changes in the base word, when the suffixes are added, the word is regular – the only issue you can encounter here is the choice between suffixes -er and -or: both may seem okay, but in this case, -er is the correct choice.

Definition of protesters:
Noun, plural of protester
1. A person who vocally disagrees with something, usually showing it in the form of a public appearance involving shouting and carrying signs.
As a whole, the protest was calm, but there were a few protesters who started to get violent.
The protesters harshly criticized the local government’s decision to cut down the nearby forest.

Incorrect spelling


Incorrect spelling, explanation: technically speaking, this word should have the same meaning as protesters, as the suffixes -er and -or are often interchangeable and often the choice is up to the speakers. When you think about it, many words with the suffix -or are very official: professor, dictator, doctor. These words are of Latin origin – and protest is too, but it was actually introduced into English from French, which is why the -er remained unchanged.