Identify the segment in the sentence which contains a grammatical error.

Two elderly ladies narrated an rather improbable story to the police.

narrated an rather

The grammatical error in the sentence "Two elderly ladies narrated an rather improbable story to the police" is "an rather". The article "an" is used before words that start with a vowel sound. The word "rather" starts with the consonant sound "r", so the correct article is "a".

The correct sentence should be "Two elderly ladies narrated a rather improbable story to the police".