In the following sentence, one part may contain a grammatical or usage error. The sentence is divided into three parts labelled A, B and C, and part D suggests that the sentence has no error. Read the sentence and identify the part that contains an error: A. The trek is difficult B. but it is far worth C. the endeavour D. No error.

Introduction / Context:
Error spotting questions are a common feature of English sections in competitive exams. They test your knowledge of grammar, idiomatic usage, and sentence structure. In this question, the sentence about a trek is broken into parts so that you can examine each portion carefully. Your task is to decide which specific part contains an error in standard English usage or whether the sentence is correct as it stands.

Given Data / Assumptions:
- Complete sentence: The trek is difficult but it is far worth the endeavour. - Part A: The trek is difficult. - Part B: but it is far worth. - Part C: the endeavour. - Part D: The sentence is grammatically correct; no error.

Concept / Approach:
The central idea of the sentence is that although the trek is challenging, it is still very rewarding. In English, the natural and idiomatic expression is that something is well worth the effort or the endeavour. The phrase far worth is not a standard collocation. While we can say far better, far more, or well worth, we do not say far worth. Therefore, the issue in this sentence is one of idiomatic usage rather than basic grammar alone. We must identify which part contains this non standard phrase.

Step-by-Step Solution:
Step 1: Examine part A, The trek is difficult. This is a grammatically correct independent clause with a clear subject and verb and no error in structure. Step 2: Examine part B, but it is far worth. Here, the conjunction but introduces contrast, which is correct. However, the expression far worth is not idiomatic. The correct phrase should be well worth or worth it. Step 3: Examine part C, the endeavour. This part functions as the object of worth and is grammatically acceptable. Step 4: Since only part B contains non standard usage, the error is located in part B.

Verification / Alternative check:
Rewriting the sentence with a correct idiom confirms the mistake. The corrected sentence would be The trek is difficult but it is well worth the endeavour. Now, every part reads naturally and is grammatically sound. Because part B is the only fragment that required change, selecting that part as the error is consistent with the intended structure of the question and with standard English usage. This verification step also reinforces your understanding of the correct phrase.

Why Other Options Are Wrong:
Option A is correct because it clearly states the fact that the trek is difficult without any grammatical or usage error. Option C is correct because the phrase the endeavour serves as a proper noun phrase object and fits naturally after worth. Option D is incorrect because the sentence is not fully correct; the phrase far worth is unidiomatic and must be replaced.

Common Pitfalls:
A common difficulty in such questions is that learners may focus only on obvious grammar issues such as subject verb agreement or tense and may overlook idiomatic expressions. Another pitfall is being influenced by similar correct phrases like far better and assuming that far worth is also acceptable. Regular reading of good quality English texts and noting collocations such as well worth, worth the effort, and worth the endeavour help build a natural feel for correct expressions. Always check whether the expression you see is one you have encountered in correct usage before.

Final Answer:
The error is in part but it is far worth, which should be replaced by an expression like it is well worth.