Correct spelling


Correct spelling, explanation: protesters is a noun made from the base word protest and a suffix -er, which is usually used to create a noun with the meaning a person who…. Then the plural suffix -s is added at the end. The construction of the word is relatively easy and logical, and the -er in protesters can be considered the most standard suffix in this role.

Definition of protesters:
noun, plural of protester
1. People who protest; show their opposition to something, usually shouting and carrying signs.
Protesters went on a strike and blocked most of the important streets in the city center.
Some of the protesters are intense, but it shows their dedication.

Incorrect spelling


Incorrect spelling, explanation: protestors is an incorrect word, despite being created on a correct principle. The suffix -or used here can be indeed used to suggest the meaning of a person who protests, but there is more nuance to using this suffix: it’s usually used with words of Latin origin. The -er suffix has a wider use and can be used here, thus making protesters the correct word.