The correct answer is Himachal Pradesh.

• World's tallest post office is located in Himachal Pradesh.
• Hikkim village has a post office situated at an elevation of 4,400 m (14,400 ft) and this post office is the highest post office in the world.
• The post office connects small villages in this isolated region to the rest of the world.
• It also acts as a savings bank where villagers can deposit money in their savings accounts or withdraw money.
• Intrepid travellers make it this far to Hikkim village to take pride in mailing their letters from the highest post office on Earth. 

• India has the most number of Post offices in the world with more than 1.5 lakhs post offices.
• China ranked second in the most number of Post offices.
• Indian post established on 1 April 1854.
• Warren Hastings had taken initiative under the East India Company to start the Postal Service in India in 1788.
• It was initially established under the name "Company Mail".
• It was later modified into service under the "crown" in 1854 by Lord Dalhousie.