Answer:
I'd say the 4th one!
Explanation:
We can rule out the 1st option, as there are several rocks which are less dense than the black rocks mentioned.
The 2nd one cannot be inferred from only one sample of a green rock!
The 3rd one is simply false, as there's a pink rock which has a density higher than 2.7.
So, we are left with the 4th option! :)
