In the following sentence with multiple blanks about literature and social protest, select the word that best completes the first blank: "Literature is a _____ through which a person conveys his ideas or protests against different norms of society. The words that deal with a moral issue are of particular importance in literature. They are written with a particular purpose in mind."

Introduction / Context:
This question belongs to a set based on a single paragraph about literature, social norms and moral issues. The paragraph contains several blanks, each tested in a separate item. Here, the focus is on the first blank in the sentence Literature is a _____ through which a person conveys his ideas or protests against different norms of society. The aim is to choose the word that best describes the role literature plays as a means of communication and expression.

Given Data / Assumptions:

• The full reconstructed sentence is: Literature is a _____ through which a person conveys his ideas or protests against different norms of society.
• Literature here refers to novels, poems, plays and other written works.
• The sentence suggests that literature is a means or channel of communication.
• The options given are base, medium, source, subject and channel.

Concept / Approach:
When we speak about literature as a way by which ideas are communicated, we usually use words like medium or means. The structure through which already implies a means or channel, so the word that fits most naturally is medium. The other options either change the intended meaning or do not collocate so well. A base is a foundation rather than a means of communication. A source may refer to the origin of ideas but not to the method by which they are conveyed. Subject refers to the topic of writing, not its communicative role. Channel is close in meaning but sounds less idiomatic in the fixed phrase literature is a medium through which.

Step-by-Step Solution:
Step 1: Focus on the phrase through which a person conveys his ideas or protests, which clearly points to a means of communication.
Step 2: Identify which option commonly appears in the expression is a medium through which.
Step 3: Note that literature is frequently described as a medium of expression in academic and critical writing.
Step 4: Compare the options and see that base, source and subject do not fit well with through which a person conveys his ideas.
Step 5: Select medium as the word that best completes the first blank in both sense and collocation.

Verification / Alternative check:
If we read the sentence with the chosen word, Literature is a medium through which a person conveys his ideas or protests against different norms of society, it sounds natural and is frequently found in literary criticism. Replacing medium with base would give Literature is a base through which, which is unusual and awkward. Using source shifts the focus to the origin of ideas rather than the method of communication. Subject would change the meaning to topic, and channel, although close, is less standard in this fixed academic phrase. Therefore, medium is the most accurate and idiomatic choice.

Why Other Options Are Wrong:
Option A, base, refers to a foundation or starting point and does not express the idea of a communicative path. Option C, source, points to the origin of something but not necessarily to the way ideas are transmitted to others. Option D, subject, refers to what the literature is about rather than how it conveys ideas. Option E, channel, can be used with through, but literature is more commonly described as a medium rather than a channel in formal contexts. Hence, these alternatives do not match the intended meaning as precisely.

Common Pitfalls:
Some learners may be tempted by source or channel because they also relate to communication, but they overlook typical academic collocations. Reading literary essays and criticism can help build familiarity with phrases like medium of expression, medium of communication and literary medium. Recognising these common word partnerships is a powerful way to approach vocabulary based blanks in competitive exams.

Final Answer:
The best word to complete the first blank is medium, giving the sentence: Literature is a medium through which a person conveys his ideas or protests against different norms of society.