The ex-officio chairman of the Zonal Council is the Union Home Minister. Let's understand why this is the correct answer in detail:

Zonal Councils are regional advisory bodies created by an act of Parliament to promote cooperation and coordination among the states and union territories in India. These councils were established under the States Reorganization Act, 1956, to address issues of common interest and ensure balanced socio-economic development.

There are five Zonal Councils in India, each covering a specific zone:
1. Northern Zonal Council: Comprising the states of Haryana, Himachal Pradesh, Jammu and Kashmir, Punjab, Rajasthan, and the Union territories of Delhi and Chandigarh.
2. Central Zonal Council: Comprising the states of Chhattisgarh, Madhya Pradesh, Uttar Pradesh, and Uttarakhand.
3. Eastern Zonal Council: Comprising the states of Bihar, Jharkhand, Odisha, and West Bengal.
4. Western Zonal Council: Comprising the states of Goa, Gujarat, Maharashtra, and the Union territories of Daman and Diu, and Dadra and Nagar Haveli.
5. Southern Zonal Council: Comprising the states of Andhra Pradesh, Karnataka, Kerala, Tamil Nadu, Telangana, and the Union territory of Puducherry.

The Zonal Councils are headed by the Union Home Minister, who serves as the ex-officio chairman. The Union Home Minister is a member of the Union Cabinet and is responsible for the overall administration of the country's internal affairs, including law and order, internal security, and coordination among different states and union territories.

The choice of the Union Home Minister as the ex-officio chairman is based on the need for a central authority to oversee and facilitate the functioning of the Zonal Councils. As the head of the Ministry of Home Affairs, the Union Home Minister is well-positioned to provide guidance, support, and coordination to the Zonal Councils in addressing issues of common interest and resolving inter-state disputes.

In conclusion, the ex-officio chairman of the Zonal Council is the Union Home Minister. This choice ensures effective coordination and cooperation among the states and union territories in the respective zones, promoting balanced development and addressing common issues.