**Core Concept**
Type II hypersensitivity is a type of immune-mediated reaction characterized by the binding of antibodies to specific antigens on the surface of cells or tissues, leading to their destruction or functional impairment. This type of hypersensitivity involves the activation of effector cells, such as neutrophils and macrophages, and the release of inflammatory mediators.

**Why the Correct Answer is Right**
In the context of blood transfusion, Type II hypersensitivity occurs when pre-existing antibodies in the recipient's blood react with antigens on the surface of transfused red blood cells. This antigen-antibody complex triggers the activation of complement and the release of inflammatory mediators, leading to the destruction of transfused red blood cells. This reaction can result in a severe hemolytic transfusion reaction, characterized by symptoms such as fever, chills, and hemolysis.

**Why Each Wrong Option is Incorrect**
**Option B:** Arthus reaction is a classic example of a Type III hypersensitivity reaction, which involves the formation of antigen-antibody complexes that deposit in tissues and activate complement, leading to inflammation and tissue damage.
**Option C:** Hay Fever is an example of a Type I hypersensitivity reaction, which involves the release of histamine from mast cells and the activation of eosinophils, leading to symptoms such as rhinitis, conjunctivitis, and asthma.
**Option D:** Glomerulonephritis can be caused by various mechanisms, including Type III hypersensitivity reactions, but it is not a classic example of Type II hypersensitivity.

**Clinical Pearl / High-Yield Fact**
Be aware that blood transfusion reactions are a leading cause of morbidity and mortality in medical settings, and accurate typing and cross-matching of blood products are essential to prevent these reactions.

**✓ Correct Answer: A. Blood transfusion reaction**