In the following sentence, choose the alternative that best improves the bracketed part. With a population of over one billion, India is the second (most popular countries) in the world.

Introduction / Context:
This sentence compares India with other countries on the basis of population size. It reads, "With a population of over one billion, India is the second (most popular countries) in the world." The bracketed phrase is incorrect because "popular" refers to likability, while the context clearly demands a word about population. We must choose the correct adjective and structure to express that India has the second highest population in the world.

Given Data / Assumptions:

• India's population is over one billion.
• The sentence claims that India is the second something in the world.
• The bracketed phrase "most popular countries" is clearly illogical.
• Options include:
  • more popular country
  • most populous country
  • much popular countries
  • No improvement
• The intended comparison is about number of people, not popularity.

Concept / Approach:
We must:

• Choose between "popular" (liked, well known) and "populous" (having a large population).
• Ensure singular or plural form agrees with "India is the second..."
• Ensure that the superlative "most" is used correctly in relation to "second".

In standard English, we say "second most populous country" when ranking by population.

Step-by-Step Solution:
Step 1: Replace "popular" with "populous", because we are talking about the size of the population, not fame or likability. Step 2: The structure should be "the second most populous country in the world". The noun "country" must be singular to match "India is the second ... country". Step 3: Option B "most populous country" gives precisely the correct adjective and noun in singular form. Step 4: Option A "more popular country" is wrong in both adjective choice ("popular") and comparative structure; "second more popular country" is not standard English. Step 5: Option C "much popular countries" is ungrammatical and uses the wrong adjective and number. Step 6: "No improvement" is wrong because the original phrase uses "popular countries", which is clearly incorrect in context.

Verification / Alternative check:
Insert option B into the full sentence: With a population of over one billion, India is the second most populous country in the world. This matches the well known factual statement and uses correct comparative ranking language. It is both grammatically and semantically accurate.

Why Other Options Are Wrong:

• more popular country: "Popular" refers to being liked, not population; "more popular" also does not collocate with "second" in this way.
• much popular countries: Incorrect combination of "much" with "popular" and wrong plural "countries".
• No improvement: Retains "most popular countries", which conflicts with the idea of population size.

Common Pitfalls:
Some learners misread "popular" as "populous" and overlook the error. Others may not notice that "countries" is plural while India is a single country. In grammar and usage questions, always check that adjectives precisely match the intended meaning and that singular or plural forms are correct. Knowing pairs like "industrial" versus "industrious" and "popular" versus "populous" is very important in competitive exams.

Final Answer:
The correct improved phrase is most populous country, so the correct option is "most populous country".