Let's analyze each statement individually:

Statement-I: Marsupials are not naturally found in India.

This statement is correct. Marsupials are a type of mammal where the female typically carries and nurses her young in a pouch, such as kangaroos or koalas. The majority of marsupial species live in Australia and surrounding islands, while others can be found in the Americas. However, they are not naturally found in India.

Statement-II: Marsupials can thrive only in montane grasslands with no predators.

This statement is incorrect. Marsupials are found in a variety of habitats, ranging from forests to deserts, not just montane grasslands. Moreover, they have various predators such as foxes, large birds, and crocodiles. Thus, the idea that they can thrive only in the absence of predators is not accurate.

Now, let's evaluate the options:

Option 1: Both Statement-I and Statement-II are correct, and Statement-II is the correct explanation for Statement-I.
This option isn't valid because, as we discussed, Statement-II is incorrect.

Option 2: Both Statement-I and Statement-II are correct, and Statement-II is not the correct explanation for Statement-I.
Again, this option is dismissed because Statement-II is incorrect.

Option 3: Statement-I is correct but Statement-II is incorrect.
This option confirms our analysis of the individual statements.

Option 4: Statement-I is incorrect but Statement-II is correct.
This option is also invalid because Statement-I is correct and Statement-II is not.

Hence, the correct answer is Option 3: Statement-I is correct but Statement-II is incorrect.