The restoration of the medieval cathedral took much longer than initially planned, ---- due to unforeseen structural decay in the vaulting.

Answer and explanation

Correct answer: C. largely

  1. Step 1: The sentence explains the main reason for the construction delay.
  2. Step 2: 'Largely due to' is a standard academic collocation meaning 'mostly or primarily because of'.
Answer
largely.
Why not the others: 'Merely' means only or insignificantly; 'Scarcely' means hardly; 'Barely' means almost not.

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