The correct answer for this question is option 'C', which is "Either could be used here." This means that both options 'a' (And) and 'b' (That) can be used in the given sentence.

Here is the explanation of why either option can be used:

1. Option 'a' (And):
- The word "And" is a conjunction that can be used to connect two ideas or phrases.
- In this sentence, using "And" would imply that the Kaziranga National Park is a World Heritage Site and it also hosts two-thirds of the one-horned rhinoceros in the world.
- This usage implies that the fact that the park is a World Heritage Site is connected to the fact that it hosts two-thirds of the one-horned rhinoceros.

2. Option 'b' (That):
- The word "That" is a relative pronoun used to introduce a clause that provides more information about the noun it refers to.
- In this sentence, using "That" would imply that the Kaziranga National Park is a World Heritage Site and that it hosts two-thirds of the one-horned rhinoceros in the world.
- This usage implies that the fact that the park is a World Heritage Site is a separate piece of information from the fact that it hosts two-thirds of the one-horned rhinoceros.

3. Either could be used here:
- Both options 'a' and 'b' are grammatically correct and convey slightly different meanings.
- Using "And" connects the two pieces of information more closely, implying a stronger connection between the park being a World Heritage Site and hosting the rhinoceros.
- Using "That" separates the two pieces of information, implying that the park being a World Heritage Site is a separate detail from hosting the rhinoceros.
- Since both interpretations can be valid in this context, either option 'a' or 'b' could be used.

In conclusion, option 'C' is the correct answer because either option 'a' (And) or 'b' (That) could be used in the given sentence. Both options are grammatically correct and convey slightly different meanings.

Either could be used here