💡 Detailed Explanation
The correct choice is Option 1: 'Excuse me. May I come in?' This is a highly polite expression appropriate for asking a superior (like a teacher) for permission to enter their office ('〜てもよろしいでしょうか' - is it alright if I...?). Option 2 ('どうぞ、お入りください' - Please, come in) is said by the person ALREADY inside the room. Option 3 ('ごめんください。誰かいませんか' - Excuse me, is anyone there?) is used when visiting someone's home and finding no one at the door, which is informal and inappropriate for a student entering a teacher's academic office.