each sentence is divided into four parts, in which one part may or may not contains error. Choose the incorrect part as your answer. If no such part has any error, choose ‘No error’ as your answer choice.

Known for his resilience and determination, (A) / the runner refused to let a few (B) / defeats stop him then (C) /competing in future races (D) .

The correct answer is C.

Let's analyze the sentence structure and logical flow to find the grammatical error:

1. Part (A): "Known for his resilience and determination,"
This is a past participial phrase correctly modifying the noun phrase "the runner" that follows it.

2. Part (B): "the runner refused to let a few"
This acts as the main clause with "the runner" as the subject and "refused" as the main verb. The structure "refused to let..." is grammatically sound.

3. Part (C): "defeats stop him then"
The verb "stop" is correctly used as an infinitive without "to" following the object "him" after the causative/permissive verb "let" (let someone do something). However, the word "then" is incorrect here. The sentence introduces a prepositional phrase indicating prevention from doing an action, or a conjunction indicating time/condition when participating in races. Here, the verb "stop" requires the preposition "from" before a gerund (e.g., "stop him from competing..."), or optionally the temporal conjunction "when" (e.g., "...stop him when competing..."). Using "then" is grammatically incorrect and creates an ungrammatical construction before the gerund "competing".

4. Part (D): "competing in future races"
This contains the gerund phrase functioning as the object of the missing preposition "from" (or following the temporal conjunction "when").

Therefore, part C contains the error as "then" should be replaced with "from" (or "when") to make the sentence correct: "the runner refused to let a few defeats stop him from competing in future races."