The correct answer is option 'C' - Either could be used here.

Explanation:
The sentence states that the Kaziranga National Park is a World Heritage Site and also hosts two-thirds of the one-horned rhinoceros in the world. In this context, both 'and' and 'that' can be used to connect these two pieces of information.

1. Using 'and':
- When 'and' is used, it suggests that the Kaziranga National Park being a World Heritage Site is an additional piece of information about the park, which is also known for hosting two-thirds of the one-horned rhinoceros in the world.
- Example: The Kaziranga National Park is a World Heritage Site and hosts two-thirds of the one-horned rhinoceros in the world.

2. Using 'that':
- When 'that' is used, it suggests that the fact that the Kaziranga National Park is a World Heritage Site is essential to understanding why it hosts two-thirds of the one-horned rhinoceros in the world. In other words, being a World Heritage Site is a defining characteristic of the park that explains its significance as a habitat for the rhinoceros.
- Example: The Kaziranga National Park, which is a World Heritage Site, hosts two-thirds of the one-horned rhinoceros in the world.

In this particular sentence, both 'and' and 'that' can be used interchangeably without significantly changing the meaning. The choice between 'and' and 'that' depends on the writer's preference and the emphasis they want to convey.

The sentence could be rephrased as follows to illustrate the use of 'and' and 'that':
- The Kaziranga National Park is a World Heritage Site and (it) hosts two-thirds of the one-horned rhinoceros in the world.
- The Kaziranga National Park, which is a World Heritage Site, hosts two-thirds of the one-horned rhinoceros in the world.

Therefore, as both 'and' and 'that' can be used in the given sentence, the correct answer is option 'C' - Either could be used here.

The sentence provided is missing a word that connects the two clauses. Let's analyze the options given and determine which one is correct.
Option A: "and"
- This conjunction is used to connect two similar ideas or clauses. It is commonly used to link items in a list or to join two independent clauses.
- In this case, "and" would correctly connect the fact that the Kaziranga National Park is a World Heritage Site to the fact that it hosts two-thirds of the one-horned rhinoceros in the world.
- Example: The Kaziranga National Park is a World Heritage Site, and it hosts two-thirds of the one-horned rhinoceros in the world.
Option B: "that"
- This relative pronoun is used to introduce a subordinate clause that provides additional information about the noun or noun phrase that precedes it.
- In this case, "that" would introduce a subordinate clause that provides additional information about the Kaziranga National Park being a World Heritage Site.
- Example: The Kaziranga National Park, that hosts two-thirds of the one-horned rhinoceros in the world, is a World Heritage Site.
Option C: "Either could be used here"
- This option suggests that both "and" and "that" could be used in the sentence to connect the two clauses.
- It implies that the choice between "and" and "that" depends on the intended meaning or emphasis.
- Example 1: The Kaziranga National Park is a World Heritage Site, and it hosts two-thirds of the one-horned rhinoceros in the world. (Emphasizes the connection between the two clauses)
- Example 2: The Kaziranga National Park, that hosts two-thirds of the one-horned rhinoceros in the world, is a World Heritage Site. (Emphasizes the additional information about the park)
Conclusion:
Based on the options provided, option C is the correct answer. Both "and" and "that" can be used to connect the two clauses in the sentence, depending on the intended meaning or emphasis.