Answer:
a. Formal
Step-by-step explanation:
Based on the information that is being provided regarding the situation it can be said that the tone of this message should be formal. This is mainly because the invitation that is being sent is ultimately for a job offer. Regardless of the fact that it is for an internship, this is a prestigious offer that not everyone has access to and opens the door for a full-time job position within the company, thus starting a career. Therefore it should be a formal invitation.