In the restorative justice passage, choose the correct preposition for the blank: It is not a substitute __________ the formal criminal justice system, but a good backup to reduce its workload.

Introduction / Context:
Preposition choice is a core part of English grammar tested in many exams. Certain nouns form fixed patterns with particular prepositions. The phrase "a substitute for" is a standard expression meaning something that replaces something else. This question requires recognition of that common collocation in a passage discussing restorative justice and the criminal justice system.

Given Data / Assumptions:

Sentence: "It is not a substitute __________ the formal criminal justice system, but a good backup to reduce its workload."

Subject "It" refers to restorative justice.

We know restorative justice complements, but does not replace, the formal criminal justice system.

The options are "to", "for", "of", and "from".

Concept / Approach:

In English, we say "substitute for something" or "a substitute for sugar", for example.

"Substitute to", "substitute of", and "substitute from" are not correct in this structure.

Therefore, the preposition "for" is the one that correctly completes the phrase.

Step-by-Step Solution:
Step 1: Identify the key phrase: "a substitute __________ the formal criminal justice system". Step 2: Recall the common pattern "a substitute for something". Step 3: Evaluate option a, "to". The phrase "substitute to" would be incorrect in standard English. Step 4: Evaluate option b, "for". The phrase "a substitute for the formal criminal justice system" is grammatically correct and idiomatic. Step 5: Evaluate option c, "of". "Substitute of" is not used in this way; it might appear in certain technical uses, but not in this general sense. Step 6: Evaluate option d, "from", which does not fit with "substitute" in this context. Step 7: Conclude that "for" is the only correct preposition to complete the sentence.

Verification / Alternative check:
Grammar references and dictionaries give examples such as "margarine is a substitute for butter" or "this method cannot be a substitute for hard work". Applying the same pattern, we get "not a substitute for the formal criminal justice system". Thus, option b matches standard usage.

Why Other Options Are Wrong:
Option a is wrong because "substitute to" does not form a recognised collocation in English. Option c is wrong as "substitute of" does not convey the idea of replacement in ordinary usage. Option d is wrong because "substitute from" is ungrammatical in this structure.

Common Pitfalls:
Learners sometimes treat prepositions as interchangeable and pick one by guesswork, which leads to errors. It is important to learn nouns like "substitute", "reason", "result", and their fixed prepositional patterns as complete units. Reading authentic English texts helps build an intuitive sense for these combinations.

Final Answer:
The correct answer is: for.