With cloning technology, scientists are approaching what has long been

A. achieving a consistency of quality and production in farm animals as once thought to be

Because "in farm animals" follows "production" this version seems to convey that production is in farm animals.

Also, the use of "as" does not make sense. While "as" is used in expressing certain types of comparisons, this sentence does not express a comparison. It is about one thing, a consistency of quality and production once thought to be limited to manufactured goods. The consistency of quality and production achieved in farm animals is not compared to the consistency of quality and production once thought to be limited to manufactured goods. The farm animal part and the manufactured goods part are both used in describing one consistency of quality and production. So, there is no reason for the presence of "as." All (A) does is throw in "as," as if the sentence is meant to express a comparison, without adjusting the rest of the wording in a way such that the sentence ends up exressing a comparison. So, (A) is an illogical mishmash, that, if read quickly, might seem express some kind of comparison, but really doesn't effectively express anything.

Go back and look at (A)? Does it clearly compare two things? What two things? What purpose does "as" serve?

C. achieving in farm animals a consistency of quality and production that was once thought to be

On the other hand, in (C), the modifiers and what they modfify are laid out in a way such that (C) effectively expresses a logical meaning.

The version created via the use of (C) expresses that the goal is the following:

achieving - achieving in what? - in farm animals - achieving what in farm animals? - a consistency of quality and production - what kind of consistency? - a consistency that was once thought to be limited to manufactured goods

So, (C) is laid out well to effectively express a logical meaning.