The answer here is A. Let me explain.

The structure of the sentence is: Aerugo is [X], and is [Y].
where x = “the green bloom visible on many copper items”
y = “produced over the course of time”

Don’t confuse this with the sentence structure that most people would see initially:
Aerugo is the green “bloom” [X], and [Y]
where x = visible on many copper items
y = is produed

At first glance, there may seem like something is wrong with A just because of the way they underline the sentence.
But if you step back and look at what the real structure of the sentence is, you’ll notice the keyword AND. Look to the left and look to the right.

On the left you should see “is the green bloom visible on many copper items” and on the right you should see “is produced over the course of time.”

This is consistent.

All choices (B), (C), and (D) are incorrect because the end of the answer choices use the active verb “Produces” or “that produces” or “which produces”—you need to ask yourself: “Produce what?”
Well, if you sub it into the sentence, you’ll see: “that produces, like iron rust, over the course of time”
That doesn’t make sense! You don’t just “produce over the course of time.” You need to produce something. You need to produce [x].
Since it doesn’t make sense, all of them (B, C, and D) don’t make sense.

Now, (A) does work because it does not follow this structure. Rather, it says the Aerugo “is produced, like iron rust, over the course of time.” This makes sense. Combine that with our analysis above with the X & Y consistency and you’ll see that answer (A) is correct.

Hope this helps!