The correct answer is option (A):
harvesting … potable
The correct answer is "harvesting … potable". Let's break down why this is the best fit and why the others don't work as well.
The sentence is about the steps involved in using rainwater and wastewater. The goal is to make the water reusable.
"Harvesting" rainwater means collecting it, often from rooftops or other surfaces. This is a common first step in using rainwater. "Potable" means suitable for drinking. So, the sentence is saying that research should be done on collecting rainwater and treating wastewater so that the water is safe to drink. This makes perfect sense in the context of recycling and reusing water resources.
Let's look at why the other options are less suitable:
* "Preserving … available": While preserving rainwater is important, "preserving" doesn't necessarily describe the initial step of getting the rainwater. "Available" is a general term, but not as specific as "potable" for describing water quality.
* "Controlling … possible": "Controlling" rainwater isn't as natural a fit as "harvesting." "Possible" is vague. The goal isn't just to make the water *possibly* usable, but actually *safe to drink* or use.
* "Maintaining … edible": "Maintaining" doesn't fit the context of the first part. "Edible" is a slightly less appropriate word to use for water, although in general, it can be true.
* "Collecting … accessible": "Collecting" is fine, but "accessible" is less specific than "potable". While accessible water is important, the ultimate goal here is to make the water safe for consumption.
Therefore, "harvesting ... potable" provides the most logical and specific meaning.