In the following passage some of the words have been left out. Read the passage carefully and choose the correct answer for the given blank out of the four alternatives: It so happens that when siblings sing one of _____ invariably has a less _____ vocal prowess even though _____ have a similarly commendable _____ base. Siblings sing one of _______ (second blank).

Introduction / Context:
This is a continuation of the same cloze passage. Now the focus is on the second blank in the clause: "when siblings sing one of _____ invariably has a less _____ vocal prowess". You must choose the pronoun that correctly completes the phrase "one of _____" from among "they", "their", "those", and "them". This requires a good understanding of English pronoun patterns.

Given Data / Assumptions:

- Phrase in question: "one of _____ invariably has". - The noun referred to before the blank is "siblings" (plural persons). - The verb after the blank is "has", which agrees with the singular "one". - The missing word must fit grammatically with "one of".

Concept / Approach:
The structure "one of" is normally followed by a plural pronoun in the objective case when the reference is to people already mentioned. Common examples are "one of them" and "one of us". We do not say "one of they", "one of their", or "one of those" when we are directly substituting for a personal object pronoun referring to a previously mentioned plural group. Since "siblings" has already been mentioned, we are essentially saying "one of them" (meaning one of the siblings). Therefore, "them" is the correct choice.

Step-by-Step Solution:
Step 1: Focus on the immediate phrase: "one of _____". Step 2: Test each option: "one of they", "one of their", "one of those", "one of them". Step 3: Recognise that standard English uses "one of them" to refer to one member of a previously mentioned group of people. Step 4: Substitute "them": "when siblings sing one of them invariably has a less _____ vocal prowess". Step 5: Check that the sentence now reads naturally and grammatically.

Verification / Alternative check:
Insert "them" into the partial passage: "It so happens that when siblings sing one of them invariably has a less impressive vocal prowess even though they have a similarly commendable base." This is a smooth and idiomatic sentence describing a common reality about singing siblings. Trying other options quickly reveals how awkward or incorrect they are. "One of they" is grammatically wrong; "one of their" is incomplete without a following noun; "one of those" would normally require a noun ("one of those siblings") and does not fit as a stand-alone pronoun here.

Why Other Options Are Wrong:
Option A, "they", gives "one of they", which is incorrect because "they" is a subject pronoun and cannot follow "of" in this construction. Option B, "their", is a possessive determiner and would require a noun after it, such as "their voices"; "one of their" by itself is incomplete. Option C, "those", would normally be followed by a noun ("those siblings") and is not the standard way to refer back to the group already designated as "siblings" in this context.

Common Pitfalls:
English learners often struggle with pronoun cases after prepositions such as "of". Remember that object pronouns (me, us, them) typically follow prepositions. Phrases like "one of us", "one of them", and "one of you" are extremely common. Confusing subjective, possessive, and demonstrative forms in such patterns is a frequent exam error. Practising these fixed expressions will make cloze tests like this much easier.

Final Answer:
them