In this English vocabulary question, out of the four alternatives given, choose the one word that best substitutes the expression not able to produce children and clearly describes a person who is unable to have biological offspring.

Introduction / Context:
Many English vocabulary questions in competitive exams test a learner on one word substitutions. Instead of using a long phrase such as not able to produce children, the exam asks for a single precise word that captures the same meaning. This question checks your knowledge of medical and biological terminology that is often used in both everyday language and formal writing. Choosing the right option requires you to distinguish between words that relate to fertility, pregnancy, and general productivity, and to identify which of them specifically describes the inability to have children.

Given Data / Assumptions:

• The phrase to be replaced is not able to produce children.
• We must choose a single English word that accurately expresses this meaning.
• Four alternatives plus one extra option are given, and only one is correct.
• The context refers to human fertility and the ability to have offspring.

Concept / Approach:
The key idea is to recall technical and semi technical terms related to fertility. A word that means fertile or productive will be the opposite of what we need. A word that means pregnant describes a temporary state rather than permanent inability. Therefore, the correct answer must refer to a permanent or inherent inability to produce children. The word sterile is used in medicine and biology to describe an organism, person, or animal that cannot reproduce. It can also describe soil that cannot grow plants or environments free from germs, but in this question the sense of infertility is important. By contrast, words such as fecund and prolific suggest high fertility and abundant production, so they do not match the phrase in the stem.

Step-by-Step Solution:

Verification / Alternative check:
We can verify our choice by consulting standard dictionary meanings in memory. Sterile is commonly defined as infertile, unable to produce offspring. In contrast, fecund and prolific are synonyms for highly fertile. Gravid refers to being pregnant, which clearly contradicts the idea of being unable to produce children. Hebetic is related to adolescence or the period around puberty, and not to infertility. Because sterile alone matches the phrase not able to produce children, our selection is consistent and logically sound. If you substitute the word sterile into the original phrase, the sentence retains the intended meaning, which is an additional confirmation.

Why Other Options Are Wrong:
Fecund means highly fertile or capable of producing many offspring or results, which is the opposite of the required meaning, so it is incorrect. Gravid means pregnant, especially in medical contexts, which indicates the presence of a child rather than an inability to have one, so it does not match. Hebetic relates to puberty or the stage of adolescence and has no direct connection with fertility or infertility, so it is not appropriate. Prolific refers to producing abundant works, results, or offspring and again implies high productivity, not inability, which makes it unsuitable as a substitute for the phrase in the question.

Common Pitfalls:
Candidates often confuse sterile with words that simply describe a lack of interest or dullness, because sterile can also metaphorically mean unproductive. Another common mistake is to choose fecund or prolific because they are familiar academic words, even though their meaning is the opposite of what the question demands. Some learners also mistakenly believe that gravid means barren because it is less commonly used, but in fact it indicates pregnancy. To avoid such errors, always focus on the exact meaning of the phrase in the stem and relate each option to that meaning before answering.

Final Answer:
The correct one word substitute for not able to produce children is Sterile.