An informal writing style in nonfiction would contain

Informal is defined as relaxed, unofficial, or easygoing. If this is a multiple choice question, consider what choices are formal versus informal. Anything that would fall under the category of "formal," which can also mean academic, rigid, proper, precise, or strict, can be ruled out as wrong answers. Things that would be "formal" include tones, or the way that the story is written, that are very rigid and academic. In addition, a list of facts presented one after the other would be formal, not informal. Once you are done ruling these formal answers out, you will know what is informal.