The correct option is C Resignation from the post of...
Subject of the letter contains the central idea of any letter. Resignation letter is a kind of a formal letter which one writes to one's supervisor at the time of leaving an organization. So, the subject should define the purpose of the letter. Option C is correct as it clearly denotes the purpose of the letter. The other options are not correct because they do not adhere to the norms of the format. 'Please' suggests request which is usually written in the body. So, option D is incorrect. Here, Diana is issuing her resignation from the company and not from her post. So, option B is incorrect. This is not a 'leave application'. Hence, option A is also incorrect.