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There are two issues in this question: MEANING and MODIFIERS.

From a meaning perspective, what is the thing that modern husbandry is trying to achieve? We can't achieve objects like "farm animals," we can only achieve abstract goals like "consistency." Eliminate B, D, and E.

In A and C, there is no significant difference between saying "achieving a consistency in farm animals" vs. "achieving in farm animals a consistency." The placement of the prepositional phrase does not change the meaning. The primary difference is between "as once thought to be" vs. "that was once thought to be."

The consistency was thought to be limited to manufactured goods, so we want to make sure that we're MODIFYING consistency. An "as" would be providing a comparison, but would not be modifying "consistency." We need the relative pronoun "that" to set up the subordinate clause to modify "consistency."