Concept:To choose the correct question tag, it must match the tense and the auxiliary verb of the main statement.
Explanation:The complete statement is "It has been a wonderful day."
The sentence is positive and uses the present perfect tense.
The auxiliary verb is "has".
A positive statement takes a negative question tag.
So the tag should repeat the auxiliary "has" and become "hasn't it?".
Option B uses "hadn't", which is for the past perfect tense.
Option C uses "isn't", which is for the present simple.
Option D uses "wasn't", which is for the past simple.
Only option A correctly matches the present perfect auxiliary "has".
Answer:A. hasn't it