**Core Concept**
The blast injury is caused by the rapid transmission of pressure waves through the air, resulting in damage to the lung and other internal organs. The blast wave's effects are influenced by the organ's anatomical position, blood pressure, and the ability to withstand the pressure changes.

**Why the Correct Answer is Right**
The brain is the organ least vulnerable to the blast wave due to its bony protection and the ability of the cerebrospinal fluid to cushion the brain. The skull acts as a shock absorber, reducing the impact of the pressure wave on the brain. Additionally, the brain's internal pressure is relatively stable compared to other organs, making it more resistant to the effects of the blast wave.

**Why Each Wrong Option is Incorrect**
**Option A:** The heart is highly vulnerable to the blast wave due to its exposed position and the potential for cardiac arrest caused by the rapid changes in blood pressure.
**Option B:** The lungs are highly susceptible to the blast wave, leading to pulmonary barotrauma, including alveolar rupture and hemorrhage.
**Option C:** The abdominal organs, such as the liver and spleen, are also vulnerable to the blast wave, which can cause hemorrhage and organ rupture due to the rapid pressure changes.

**Clinical Pearl / High-Yield Fact**
In blast injuries, the "Primary Blast Injury" is caused by the initial shockwave, while the "Secondary Blast Injury" is caused by flying debris and the "Tertiary Blast Injury" is caused by the victim being thrown by the blast wave.

**Correct Answer:** C. The brain.