Change the degree of comparison from superlative to comparative without changing the meaning:

Australia is the greenest island in the world.

• The comparative degree of green is green + -er , i.e., greener. 
• In option B, the superlative degree has been replaced with the right comparative degree, and therefore, the meaning of the sentence remains identical. 
• The sentence within the question says that Australia has the foremost greenery in the world i.e. it's the greenest island in the world. 
• Option B to says the identical thing employing a comparative degree, it says Australia is greener than all other islands within the world. Hence, the core meaning remains identical.
• Option A, to has the identical core meaning but, it doesn't use any comparative degree. Green may be a positive degree. 
• Statement in option C, is inappropriate as more is employed when the positive and comparative degrees are identical.
• Here, greener which is that the comparative degree with a suffix -er added to green, has been preceded by more, which is inaccurate.

Hence, option B is the correct answer.