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 _____ siblings sing one of _____ invariably has a less _____ vocal prowess even though _____ have a similarly commendable _____ base. It so happens that ______ (first blank).

Introduction / Context:
This question is based on a short cloze passage in which several words are missing. You are asked to fill only the first blank correctly. The sentence reads, in corrected spelling: It so happens that _____ siblings sing one of _____ invariably has a less _____ vocal prowess even though _____ have a similarly commendable _____ base. It so happens that ______. Here we focus only on the first blank after "It so happens that". The options are "how", "when", "if", and "why".

Given Data / Assumptions:

- Fixed opening phrase: "It so happens that". - Immediate continuation: a clause describing a situation involving siblings singing. - The passage describes a general tendency or typical situation, not a specific question. - The blank must be filled by one of: how, when, if, why.

Concept / Approach:
The phrase "It so happens that" is typically followed by a statement describing a circumstance, event, or situation, for example: "It so happens that when we meet, we talk about politics." Among the given options, "when" fits naturally to introduce a time-based clause that describes a recurring situation. "How", "if", and "why" usually introduce different types of clauses: manner, conditional, or reason. In this context, the passage is about what happens on occasions when siblings sing together. Therefore, "when" is the most suitable choice for the first blank.

Step-by-Step Solution:
Step 1: Read the base structure: "It so happens that _____ siblings sing". Step 2: Test each candidate word in the blank: "how siblings sing", "when siblings sing", "if siblings sing", "why siblings sing". Step 3: Evaluate naturalness and meaning. "How siblings sing" focuses on manner; "if siblings sing" suggests uncertainty; "why siblings sing" suggests a reason. Step 4: Notice that the passage is explaining a typical situation, namely, on those occasions when siblings sing, one of them has less vocal prowess. Step 5: "When siblings sing" expresses exactly this idea of a recurring circumstance and fits smoothly after "It so happens that".

Verification / Alternative check:
Insert "when" into the sentence and read the whole portion: "It so happens that when siblings sing one of _____ invariably has a less _____ vocal prowess even though _____ have a similarly commendable _____ base." The sentence now logically describes what usually happens on occasions when siblings sing. Replacing "when" with "how", "if", or "why" makes the sentence feel awkward and less idiomatic, which confirms that "when" is the most appropriate choice.

Why Other Options Are Wrong:
Option A, "how", would yield "It so happens that how siblings sing", which is ungrammatical and does not introduce a normal clause after this expression. Option C, "if", suggests a condition ("if siblings sing") and would usually require a main clause explaining what happens in that condition; the structure here, however, already implies a description of a recurring situation. Option D, "why", would introduce a reason clause ("why siblings sing") and does not fit the idea of describing what happens in those singing situations.

Common Pitfalls:
Many candidates mechanically choose "if" after "It so happens that" without carefully reading the rest of the sentence. Others may be tempted by "why" because it is common in questions, but this is not a question; it is a descriptive sentence. A smart approach is to quietly read the sentence with each option inserted and check for both grammatical correctness and natural flow of meaning. Doing so quickly reveals that "when" is clearly better than the other alternatives.

Final Answer:
when