**Core Concept**
The World Health Organization (WHO) has established criteria for diagnosing anaemia in infants based on haemoglobin (Hb) levels. These criteria take into account the age and sex of the infant, as well as the method used to measure Hb. For infants aged 6 months, the WHO criteria provide a specific threshold for diagnosing anaemia.

**Why the Correct Answer is Right**
Anaemia in infants aged 6 months is defined as a Hb level less than 110 g/litre according to the WHO criteria. This threshold is based on the average Hb level for healthy infants of this age group. The WHO criteria are essential for identifying and managing anaemia in infants, which can lead to significant morbidity and mortality if left untreated. The criteria also help to guide public health interventions and policy decisions aimed at reducing anaemia in this vulnerable population.

**Why Each Wrong Option is Incorrect**
**Option A:** A Hb level of 100 g/litre is lower than the WHO threshold for anaemia in infants aged 6 months, but it is not the correct answer. This option is too low and would incorrectly identify anaemia in infants who do not have the condition.
**Option B:** A Hb level of 105 g/litre is also below the WHO threshold, making it an incorrect answer. This option would lead to over-diagnosis of anaemia in infants who do not require treatment.
**Option D:** A Hb level of 115 g/litre is above the WHO threshold, making it an incorrect answer. This option would lead to under-diagnosis of anaemia in infants who do require treatment.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the WHO criteria for diagnosing anaemia in infants vary by age, sex, and method of Hb measurement. Public health professionals and clinicians must be aware of these criteria to provide accurate diagnoses and effective treatment for anaemia in infants.

**✓ Correct Answer: C. 110 gm/litre**