Directions: The sentence below is split into four segments labelled (A), (B), (C), and (D). One segment contains a grammatical error. Identify the segment with the error.

(A) The ancient sanctuary was / (B) thronged by hundreds of pilgrims / (C) from across the region / (D) during the annual carnival.

The correct option containing the grammatical error is thronged by hundreds of pilgrims.

Detailed Explanation:
In English grammar, certain verbs and participial adjectives require specific prepositions depending on the context. When describing a place or space that is filled or crowded with people, animals, or objects, the verb "throng" in its passive form takes the preposition "with" instead of "by".

Analysis of the Segments:
• (A) The ancient sanctuary was: Correct subject and auxiliary verb agreement.
• (B) thronged by hundreds of pilgrims: Incorrect preposition used. The preposition "by" should be replaced with "with".
• (C) from across the region: Correct prepositional phrase indicating origin/location.
• (D) during the annual carnival: Correct prepositional phrase indicating time/event.

Corrected Sentence:
"The ancient sanctuary was thronged with hundreds of pilgrims from across the region during the annual carnival."