Substitute the underlined word-segment with most appropriate idiom. The doctors tried very hard to save his mother but they were unsuccessful.

Correct — C. The plain phrase the idiom has to replace is tried very hard, so the whole item turns on which expression means "made every possible effort".

Moved heaven and earth carries exactly that sense: to do everything in one's power to bring something about.

Dropped in, the line reads "The doctors moved heaven and earth to save his mother but they were unsuccessful". Total effort, and but still delivers the failure — the sentence keeps its shape.

Why the others are wrong

• (a)
  faced the music — Faced the music means accepted the unpleasant consequences of one's own actions. Nobody here is being called to account, and the phrase says nothing about effort.
• (b)
  drew the line — Drew the line means set a limit and refused to go past it. That is the reverse of the sentence, where the doctors stopped at nothing.
• (d)
  ran like clockwork — Ran like clockwork describes something proceeding smoothly and exactly to plan. The sentence ends in failure, so nothing in it ran to plan.

Idiom-substitution items give you a plain phrase and ask for the fixed expression that fills the same slot.

Two things decide it: meaning and fit. The idiom must mean what the phrase means, and it must survive being put back into the sentence without breaking the tense or the connector around it.

Here the connector is but, which announces a contrast between effort and outcome. An idiom about effort keeps that contrast alive. An idiom about consequences or about smooth running kills it.

Key facts

• Moved heaven and earth means made every possible effort to achieve something.
• Faced the music means accepted the unpleasant consequences of one's own actions.
• Drew the line means set a limit beyond which one refuses to go.
• Ran like clockwork means proceeded smoothly and exactly as planned.

Common traps

• Picking the idiom you recognise rather than the one that matches the underlined phrase.
• Ignoring the connector: but signals failure, so an idiom about things going to plan cannot fit.
• Reading move heaven and earth as promising success — it describes the effort, not the outcome.

No directly related past PYQ was found.