I am not really sure that I know what the answer is.
Here, you would need to add the preposition "in."
The sentence should read: My elder sister is really interested in the problems of the environment.
Other prepositions would not make sense and could confuse the reader as to the intended meaning.
warranty means your item that you have can be fixed
example: I bought a PS5 and I put warranty on it.
I break the ps5. now my ps5 can be fix or I can get a new one because I have a warranty on my ps5
