**Core Concept**
The World Health Organization (WHO) defines infertility as a couple's inability to achieve pregnancy after a specific period of unprotected sexual intercourse. This definition is crucial in clinical practice, guiding the evaluation and treatment of infertile couples.

**Why the Correct Answer is Right**
In the context of modern society, where lifestyle factors and stress levels can impact fertility, the WHO recommends a minimum duration of one year of unprotected sexual intercourse to confirm infertility. This duration allows for adequate time for ovulation, fertilization, and implantation to occur, taking into account factors such as menstrual cycle irregularities, tubal patency, and sperm quality.

**Why Each Wrong Option is Incorrect**
**Option B:** One and a half-year is an unnecessarily prolonged period, as it may delay the initiation of fertility evaluation and treatment, potentially affecting the couple's chances of achieving pregnancy.
**Option C:** Two years is excessively long, as it may lead to unnecessary emotional distress and financial burden on the couple. Moreover, it may also delay the opportunity for fertility preservation and treatment options.
**Option D:** Three years is an unrealistic expectation, as it is unlikely that a couple would be willing to wait for such an extended period before seeking medical attention for infertility.

**Clinical Pearl / High-Yield Fact**
When evaluating infertility, it is essential to consider the couple's medical history, lifestyle factors, and the duration of unprotected sexual intercourse. A thorough clinical evaluation and diagnostic testing should be initiated after a minimum of one year of trying to conceive.

**✓ Correct Answer: A. One year**