The correct option is D Respected Sir
The given letter is written by a student to his principal. Hence, the tone must be formal. Blank 3 is to be filled with an opening salutation.
Option A: 'Respected Sir' is a widely accepted formal salutation, used in applications and business letters. Here it refers to the recipient, the principle. Hence A is the correct option.
Option B is incorrect because 'Hi' is an informal greeting.
Option C is, similarly, incorrect because 'hello' is not used in formal letters.
Option D is incorrect because, while writing an application to the principle, the salutation 'dear' isn't used. It is generally used in informal letters.