Students who use text language in an online academic environment violate which rule of netiquette

The use of proper language.

Explanation:

Although there is no official guidance or rules in netiquette and it can change from a website to another, there are some universal rules. Among which there is the use of a proper language, according to the environment. If it is an academic environment, text language could appear too casual, informal or even childish, as this one is full of slangs, emojis, and other casual verbatims. Therefore, it should be avoided.