The research was carried out … a sample population.

1. Analyze the question This tests your knowledge of prepositions. You must choose the preposition that correctly follows the phrase "carried out" when talking about research and a sample population.

2. Explain the concept In academic English, we say research is carried out/conducted on a group of subjects (people, animals, or things being studied). The preposition on shows the object or subject of the study.

Examples:

• The experiment was conducted on mice.
• The survey was carried out on 500 students.

3. Work through the options

• A. over: "over a sample population" suggests across or above something, not the subject of a study. Wrong.
• B. in: "in a sample population" could work loosely (e.g., "the disease was found in the population"), but it is not the standard phrase with "carried out".
• C. with: This suggests that the sample population helped carry out the research, which changes the meaning. Wrong.
• D. on: This is the standard, correct use, meaning the sample population was what the research was done on.

4. Answer The correct answer is D. on.

The sentence reads: *The research was carried out on a sample population.

5. Exam tip When you see "carried out", "conducted", or "performed" followed by a group being studied, think on. Remember: research is done ON its subjects.