In the following passage, some of the words have been left out. Read the passage carefully and select the correct answer for the given blank out of the four alternatives. Focus on the phrase "so careless use of language __________ indulged reacts on the mind" and choose the most appropriate word to complete this conditional expression.

Introduction / Context:
This cloze passage uses an extended metaphor to compare language with air and skin, and then warns about the harmful effects of careless language on thought. The specific blank tested here appears in the clause "so careless use of language __________ indulged reacts on the mind". The examiner wants to check whether you can select the most suitable connector to express a conditional relationship between indulging in careless language and the negative reaction it causes in the mind.

Given Data / Assumptions:

• The relevant sentence in the passage reads: "so careless use of language __________ indulged reacts on the mind to make it permanently and increasingly careless, illogical, and inaccurate in its thinking."
• Options given: of, then, if, at.
• The writer is describing what happens when careless language is indulged in.
• The required word must logically connect "careless use of language" with "indulged" and show the condition under which the reaction occurs.

Concept / Approach:
The passage describes a cause and effect or conditional relationship: when (or if) careless language is indulged, it reacts on the mind and damages thinking. In standard English, a natural way to form this type of condition is with if. The phrase "if indulged" concisely means "if it is indulged in" or "if people indulge in it". The other options either do not fit grammatically or fail to express this conditional idea at all. Even though "once indulged" would also sound good, among the four options offered, if is the only one that maintains both correct grammar and the intended meaning.

Step-by-Step Solution:
Step 1: Identify the relationship expressed: careless use of language, when allowed or indulged, has a harmful reaction on the mind. Step 2: Recognise that we need a word that introduces a condition: the effect happens only under that condition. Step 3: Test "if": "so careless use of language if indulged reacts on the mind". This can be read as a condensed structure for "if it is indulged, it reacts on the mind" and clearly expresses the condition. Step 4: Test "of": "use of language of indulged" is ungrammatical and breaks the sentence; of does not fit before a past participle in this way. Step 5: Test "then": "language then indulged reacts" sounds awkward and does not correctly link indulgence and reaction as a conditional clause. Step 6: Test "at": "language at indulged reacts" is completely ungrammatical and can be rejected immediately. Step 7: Conclude that only "if" produces a grammatically acceptable and logically meaningful clause that fits the author's argument.

Verification / Alternative check:
Paraphrase the sentence with a complete conditional clause: "So if careless use of language is indulged, it reacts on the mind to make it permanently and increasingly careless, illogical, and inaccurate in its thinking." This fully expanded form is clearly correct English and matches the author's warning. Replacing "if" with any of the other options breaks the logic or grammar of the sentence. This confirms that "if" must be the correct choice for the blank.

Why Other Options Are Wrong:
"Of" is wrong because it cannot function as a conditional connector before "indulged". "Then" is wrong because it usually comes after a conditional clause (for example, "if X, then Y") and cannot by itself introduce the condition. "At" is wrong because it expresses location or time, not a condition, and does not fit before a participle in this structure.

Common Pitfalls:
Students sometimes overthink cloze questions and search for rare or technical uses of short words like of or at. However, most exam passages use standard connectors and collocations. Another common mistake is expecting the exact word they would have written (such as "once") and being confused when it does not appear among the options. In such cases, you must choose the closest grammatical and logical fit from the given options, which here is "if".

Final Answer:
The correct word to complete the blank is if, giving "so careless use of language if indulged reacts on the mind".