In this sentence improvement question, you must choose the best substitute for the underlined description. Original idea: A foolish person is someone who is easily taken in and tricked by others.

Introduction / Context:
This question asks for a precise adjective that describes a person who is easily deceived. The sentence explains that such a person is easily taken in and tricked by others. English has a specific word for this characteristic. Your task is to choose the word that expresses this idea accurately in a single term, which is a common type of one word substitution in exams.

Given Data / Assumptions:

• The described person is easily taken in by others.
• The key idea is that this person believes things too quickly and is easy to cheat.
• Options given are sober, stupid, gullible, and no improvement.
• The examination expects a precise vocabulary choice rather than a general insult.

Concept / Approach:
The adjective gullible refers to a person who is easily persuaded to believe something and is easy to deceive. It is widely used in psychological and social contexts to describe vulnerability to being tricked. Stupid is a general term for low intelligence but does not specifically focus on being easily deceived. Sober relates to not being drunk or being serious, which is unrelated. Therefore, gullible is the exact word that matches the explanation in the sentence.

Step-by-Step Solution:
Step 1: Focus on the essential description easily taken in and tricked by others. Step 2: Recall that gullible means easily deceived or too ready to believe what other people say. Step 3: Compare this definition with stupid, which refers more broadly to lack of intelligence and does not always imply being easily tricked. Step 4: Examine sober, which usually means not drunk or serious, and clearly has no link to being deceived. Step 5: Consider No improvement and note that the original wording is long and descriptive, whereas a single precise adjective is preferred. Step 6: Conclude that gullible is the best fit for the given description.

Verification / Alternative check:
Use the word in sample sentences. For example, Children are often gullible and may believe everything they see on television. This sentence conveys exactly the idea of being easily tricked. Substituting stupid would change the focus from being deceived to general intelligence, which is not the main point in the original description. This confirms that gullible is the correct choice.

Why Other Options Are Wrong:
Sober is wrong because its main meanings relate to not being under the influence of alcohol and to seriousness. Stupid is wrong because it is a broad insult about intelligence and does not specifically capture the nuance of being easy to deceive. No improvement is wrong because the sentence clearly defines a concept for which a standard adjective exists, and good style prefers concise and precise wording when possible.

Common Pitfalls:
Some candidates may choose stupid due to its negative sense, but examiners usually look for the most accurate word, not the most insulting one. Others may ignore the clue easily taken in and focus only on foolish person, missing the subtle meaning. When solving such questions, always match the definition in the stem with the dictionary meaning of the options rather than relying only on emotional tone.

Final Answer:
The correct adjective for a person who is easily taken in and tricked is gullible.