The correct option is
D Girl's school is near the post office in Ambala.
Option B - The school is of the girl's, hence we should use an apostrophe and an 's' here. As 'Post' and 'Office' are not proper nouns, we can't capitalise their first letters. Hence, option B is incorrect.
Option C - The school is of the girl's, hence we should use an apostrophe and an 's' here. As 'Post' and 'Office' are not proper nouns, we can't capitalise their first letters. Also, 'Ambala' is a proper noun and hence 'A' should be capitalised. Thus option C is incorrect.
Option D - As 'Post' and 'Office' are not proper nouns, we can't capitalise their first letters. Hence, option D is incorrect.
Option A - To show possession, we use 'Girl's school' and since 'Ambala' is a proper noun, 'A' should be capitalised. Hence, option A is the correct answer.
'Girl's shool is near the post office in Ambala.'