In the nuclear security passage, choose the correct relative pronoun to complete the clause: "___________ includes nuclear materials used in nuclear weapons".

Introduction / Context:
This question checks your understanding of relative pronouns in complex sentences drawn from a passage on nuclear security. Relative pronouns such as "which", "that", "who", and "whom" introduce clauses that describe a noun. Choosing the correct one is essential for precise and grammatically correct formal writing.

Given Data / Assumptions:

• The clause is: "___________ includes nuclear materials used in nuclear weapons".
• The wider passage refers to "the need to maintain effective security of all nuclear materials".
• Options are: which, that, those, who, whom.
• The missing word must link back to "the need" or "effective security" and introduce a descriptive clause.

Concept / Approach:
Relative pronouns link a noun to a clause that gives more information about it. "Which" is used for things, "that" can be used for people or things in restrictive clauses, "who" and "whom" are for people, and "those" is a demonstrative pronoun. The structure "which includes..." is standard when adding non defining information about a preceding noun phrase in formal writing.

Step-by-Step Solution:
Step 1: Understand that the clause "includes nuclear materials used in nuclear weapons" is describing the broader phrase "all nuclear materials". Step 2: Recognise that "all nuclear materials" is a non living, inanimate noun phrase. Step 3: The most natural relative pronoun to refer back to an inanimate noun phrase in formal style is "which". Step 4: Check "which includes nuclear materials used in nuclear weapons". This is smooth and grammatically correct. Step 5: Consider "that includes nuclear materials used in nuclear weapons". While "that" can be used, exam setters usually prefer "which" in this sort of descriptive non defining clause after a comma. Step 6: "Those includes..." is wrong because "those" is a demonstrative pronoun and would require plural verb "include", and still would not connect properly to the previous phrase as a relative clause. Step 7: "Who" and "whom" refer to people, not to materials or things, so they are inappropriate here. Step 8: Therefore, "which" is the best and most standard choice.

Verification / Alternative check:
Reconstruct the full idea: "the need to maintain effective security of all nuclear materials, which includes nuclear materials used in nuclear weapons". This is exactly how formal reports and policy papers phrase such ideas. Replacing "which" with "who" immediately sounds wrong, and "those" breaks the sentence structure. This confirms "which" as the correct answer.

Why Other Options Are Wrong:

• that: Grammatically possible in some contexts, but in formal non defining clauses after a comma, "which" is preferred and is what examinations usually expect.
• those: A demonstrative pronoun that would not correctly introduce a relative clause and would mismatch verb agreement.
• who: Used only for people or personified entities, not for materials.
• whom: Objective case of "who", again limited to people and used as an object, not as a subject introducing "includes".

Common Pitfalls:
A frequent error is to overuse "that" or to confuse "who" and "which". Some students also forget about subject verb agreement when using "those". To avoid such mistakes, remember that "which" and "that" usually refer to things, "who" and "whom" refer to people, and demonstratives like "those" do not introduce relative clauses. Reading formal reports gives you many examples of correct relative pronoun usage.

Final Answer:
The correct relative pronoun in this context is which.