The clumping together of red blood cells is

1. Analyze the question The question asks for the term that describes red blood cells sticking or clumping together.

2. Explain the concept Blood groups (A, B, AB, O) are determined by antigens on the surface of red blood cells. Plasma contains antibodies against the antigens not present on a person's own cells. When incompatible blood is mixed, for example group A blood given to a group B person, the antibodies bind to the foreign antigens and cause the red blood cells to stick together in clumps. This clumping is called agglutination.

3. Check each option

• A. Agglutination: the clumping together of red blood cells. This fits the definition.
• B. Fusion (spelt "fussion" in the question): the joining of two things into one, such as gametes fusing during fertilisation. It does not describe clumping of blood cells.
• C. Transfusion: the transfer of blood from one person (the donor) to another (the recipient). It is the process, not the clumping.
• D. Compatibility: the state in which donor and recipient blood can mix without agglutination. It is the opposite of what the question describes.

4. Answer The correct option is A, agglutination.

5. Exam tip "Agglutinate" comes from the Latin word for "glue," so think of the cells being glued together. Remember the chain: incompatible transfusion → antigen-antibody reaction → agglutination. This also explains why blood grouping and cross-matching are done before any transfusion.