Question

Which sentence is written using formal language? A.Erin has a crucial appointment early Monday morning.

B. That was a real bogus tale Charlie told the teacher.

C. Let me have a sneak-peak or I am simply gonna explode.

D. I do not think Tim will be here today; he's playing hookie.

Answer 1

A. Formal words such as "crucial" and "appointment" are used. In the others, informal words are used. These include: "bogus", "gonna" and "playing hookie".
Hope I helped. :P

Answer 2

Option A. Erin has a crucial appointment early Monday morning is written using formal language, and a formal structure as well, because it does not use contractions or slang.

The rest of the sentences include either low-register vocabulary, like sneak-peak, or informal structures, such as contractions.