In Which of the Following Messages Would It Be Appropriate

Multiple Choice

In which of the following messages would it be appropriate to use informal language?

A) A message from an HR manager to his company's CEO, requesting an appointment
B) A message written by a customer care executive to a new customer, welcoming her
C) A message written to a coworker, asking her about her plans for the day
D) A letter written to a government official, requesting for the renewal of one's work permit
E) A message written by a line manager to the CIO of the company