b. The conversation uses contractions five times.
Formal language is typically reserved for professional and academic settings such as business meetings, business emails, job applications, resumes, school essays, school discussions, etc. Informal language can appear in conversations with friends and family, texts, blogs, social media, etc. Informal language is characterized by use of slang, idioms, contractions, humor. That said, the only informal characteristic present within the conversation is the use of contractions (such as don't for do not or I'll for I will).