When deciding whether a piece of writing should be written in a formal or informal way, the author should primarily consider the of the writing. select one: a. purpose b. quality c. length d. subject

I would go with A.

It is definitely not length because that has nothing to do with it, the subject shouldn't affect formal or informal text. Quality is the closest but I think it is incorrect because a story that is written informally can have high quality.

When you compare two things without using like or as thats what it means