**Answer:**

A land-locked harbor refers to a harbor that is completely surrounded by land and has no direct access to the open sea. Among the options provided, Visakhapatnam is the only land-locked harbor. Let's understand why Visakhapatnam fits the description of a land-locked harbor and why the other options do not.

**1. Visakhapatnam:**
Visakhapatnam is a major port located in the state of Andhra Pradesh, India. It is situated on the east coast of India and is surrounded by the Eastern Ghats on one side and the Bay of Bengal on the other side. The port is located at the mouth of the River Godavari, which means it is not directly open to the sea. The presence of land on both sides makes it a land-locked harbor.

**2. Ennore:**
Ennore is a major port located in the state of Tamil Nadu, India. It is situated on the Coromandel Coast of the Bay of Bengal. Unlike Visakhapatnam, Ennore is not completely surrounded by land. It has direct access to the sea, so it is not a land-locked harbor.

**3. Mumbai:**
Mumbai is a coastal city located in the state of Maharashtra, India. It is situated on the western coast of India and is surrounded by the Arabian Sea. Mumbai has a natural deep-water harbor called the Mumbai Harbour, which provides direct access to the sea. Therefore, Mumbai is not a land-locked harbor.

**4. Haldia:**
Haldia is a major port located in the state of West Bengal, India. It is situated on the Haldia River, which is a distributary of the Ganges River. Haldia is connected to the Bay of Bengal through the Haldia Dock Complex, which allows ships to access the sea. Thus, Haldia is not a land-locked harbor.

In conclusion, among the given options, Visakhapatnam is the only land-locked harbor as it is completely surrounded by land and does not have direct access to the open sea. The other options, Ennore, Mumbai, and Haldia, all have direct access to the sea and are not land-locked harbors.