Concept:The phrase “as best they could” is a fixed idiomatic expression meaning “to the best of their ability.”Explanation:The idiom already contains the word “best,” so adding an extra “as” is redundant.Option B (“as best as they could”) is therefore not the standard accepted form.Option D (“as they could best”) uses an awkward and incorrect word order.Option A (“as they could”) fails to include “best,” so it loses the intended meaning.Thus, the correct idiomatic expression to complete the sentence is “as best they could.”Answer:C. as best they could