Option B - All the words in the given sentence are not proper nouns and hence only 'Mita's' should have a capital 'M'. Thus option B is incorrect.
Option C - 'Mitas' is incorrect, as to show someone's possession we use an apostrophe and an 'S'. For example, 'Rahul's office is neat and clean.' Also, the sentence is a simple sentence and not an exclamatory sentence. Hence, option C is incorrect.
Option D - The given sentence is not a question and hence we can't use a question mark here. Thus option D is incorrect.
Option A - 'Mita's' is used appropriately to show the ownership of her 'lost bag'. Also, the sentence ends with a full stop. Hence, option A is the correct answer,
'I found Mita's lost bag.'