**Core Concept**
Triage is a systematic process used in emergency situations to prioritize patients based on the severity of their condition and allocate limited resources accordingly. This process involves categorizing patients into different levels of urgency to ensure that those who require immediate attention receive it first. Triage is essential in disaster relief, mass casualty situations, and in healthcare settings where resources are limited.

**Why the Correct Answer is Right**
The correct answer, **B. Categorisation of the patients and treating them according to the available resource**, accurately reflects the core concept of triage. This process involves assessing patients' conditions and allocating resources based on their needs, rather than treating the most serious cases first. Triage is not just about treating the most critical patients, but also about managing resources effectively to ensure that those who can be saved receive the necessary care. This involves categorizing patients into different levels of urgency, such as red (immediate treatment), yellow (delayed treatment), green (walking wounded), and black (expectant – no treatment).

**Why Each Wrong Option is Incorrect**
**Option A:** Treating the most serious cases first is not the primary goal of triage. While it is true that critical patients require immediate attention, triage also involves managing resources and prioritizing patients based on their likelihood of survival and the availability of resources.

**Option C:** Cautery burns is a medical treatment used to close wounds, but it has no relation to the concept of triage.

**Option D:** Treating mentally ill patients is not the primary focus of triage, which is concerned with prioritizing patients based on their medical condition and the availability of resources.

**Clinical Pearl / High-Yield Fact**
A useful mnemonic to remember the priorities in triage is "RED-Y-GREEN-BLACK", where RED represents immediate treatment, YELLOW represents delayed treatment, GREEN represents walking wounded, and BLACK represents expectant – no treatment.

**✓ Correct Answer: B. Categorisation of the patients and treating them according to the available resource**