In the given passage about an ideal policeman and “good policing”, choose the correct relative word to fill the blank in the sentence: “an idea _________ even the best of criminal justice thinkers have found difficult to define.” Select the option that makes the sentence grammatically correct and natural.

Introduction / Context:
This question tests your understanding of relative pronouns in English grammar. The sentence is taken from a passage that describes “good policing” as an idea that is difficult to define. To connect the noun “idea” with the clause “even the best of criminal justice thinkers have found difficult to define”, you must choose the most appropriate relative word. This is a common pattern in English language questions in many competitive exams.

Given Data / Assumptions:

• The phrase to complete is “an idea _________ even the best of criminal justice thinkers have found difficult to define”.
• The options are “that”, “which”, “whom”, and “who”.
• The noun being described is “idea”, which refers to the concept of “good policing”.
• The relative clause describes what the thinkers have found difficult to define.

Concept / Approach:
Relative pronouns connect a noun to an additional clause that gives more information about that noun. For things and abstract concepts like “idea”, English commonly uses “that” or “which”. The pronouns “who” and “whom” are normally used for people. In restrictive or defining relative clauses, where the clause is essential to the meaning of the sentence, “that” is particularly common in modern English. Here, the clause about criminal justice thinkers is essential, so “that” is the most natural choice.

Step-by-Step Solution:
Step 1: Identify the noun being described: the word “idea”. It is an abstract thing, not a person. Step 2: Immediately rule out “whom” and “who”, because those pronouns are used for people, such as “the person who helped me”. Step 3: Consider “which”. The sentence “an idea which even the best of criminal justice thinkers have found difficult to define” is grammatically possible, and some styles of English do use “which” for things. Step 4: Consider “that”. The sentence “an idea that even the best of criminal justice thinkers have found difficult to define” is very natural and is the typical structure in modern restrictive relative clauses, especially in exam patterns. Step 5: Since the clause is essential, exam setters usually prefer “that” as the most appropriate answer. Step 6: Select “that” as the best and most exam-friendly choice.

Verification / Alternative check:
Read the complete sentence with the chosen word: “An ideal policeman is a myth. You come across him only in crime fiction. Equally elusive is ‘good policing’, an idea that even the best of criminal justice thinkers have found difficult to define.” The sentence flows smoothly and clearly. The use of “that” links the noun “idea” directly to the defining description, which is exactly the function of a restrictive relative clause.

Why Other Options Are Wrong:

• which: While not always grammatically wrong, exam patterns for restrictive clauses with abstract nouns like “idea” usually favour “that”. Between two viable options, the test expects the more standard restrictive choice.
• whom: Used for objects of verbs or prepositions referring to people, for example “the man whom I met”. It cannot refer to an “idea”.
• who: Used for subjects referring to people, as in “the teacher who explained the lesson”. It is not correct for non-human abstract nouns like “idea”.

Common Pitfalls:
One common error is to treat “which” and “that” as completely interchangeable and pick randomly. Another is to overlook the basic rule that “who” and “whom” generally refer to persons, not to things or ideas. When dealing with relative clauses in examinations, always check (a) what noun is being described and (b) whether the clause is essential to the meaning. This quickly narrows the correct choice.

Final Answer:
The correct relative word is that, giving “an idea that even the best of criminal justice thinkers have found difficult to define.”