I think option I is wrong, please help what is wrong in my understanding.
On reading the 1st paragraph, what i understood is that volcanic rocks are found on the oceanic island. The oceanic island is formed from Lower mantle. The rocks are composed of upper mantle.
It’s a hard sentence to parse AND there’s an inference which make this challenging.
What I think you’re missing is what the author is talking about when they say “whose source”. They’re talking about the midocean ridge system, not the volcanic rocks. So the point is that the rocks are different from the ridge system and the ridge system is the upper mantle.
This is a grammar thing - when we say “which” or “whose”, we mean the thing we most recently mentioned.
