Find the part of the sentence that contains an error: It was at the summit in Vienna (1)/ where the treaty was officially ratified, (2)/ thereby bringing to an end (3)/ decades of political uncertainty in the region. (4)

Correct — B.
The sentence is an it-cleft: It was + the focused phrase + a linking word + the rest.

The focus, at the summit in Vienna, is a prepositional phrase that already says where. So the link is that, not where: "It was at the summit in Vienna that the treaty was officially ratified."

The error sits in part (2), option (b).

Why the others are wrong

• (a)
  (1) — It was at the summit in Vienna is the correct opening of a cleft: It was + the phrase being spotlighted. Nothing here needs changing.
• (c)
  (3) — thereby bringing to an end is a sound participle clause giving the result of the ratification. Its long object simply follows the phrase to an end.
• (d)
  (4) — decades of political uncertainty in the region is the object of bringing to an end. The noun phrase is complete and correctly formed.

An it-cleft splits one sentence to spotlight one element. "The treaty was ratified at the summit" becomes "It was at the summit that the treaty was ratified."

The linking word in a cleft is that, or who when the focus is a person. When the focus is a place phrase with its own preposition, where would mark the place a second time.

Key facts

• It-cleft pattern: It + be + focus + that + rest of the sentence.
• A person in focus may take who: It was Rahul who called.
• Deleting It was ... that should leave a complete sentence.

Common traps

• Accepting where because the focus is a place
• Hunting for an error in "bringing to an end decades of", which is only a shifted object

No directly related past PYQ was found.