The correct answer is option 2, red blood cells. Sickle-cell anemia is a genetic disorder that affects the shape and function of red blood cells. In normal individuals, red blood cells are round and flexible, allowing them to easily navigate through blood vessels. However, in individuals with sickle-cell anemia, their red blood cells become rigid and assume a sickle or crescent shape.

This abnormal shape of red blood cells causes them to get stuck in blood vessels, leading to blockages and reduced blood flow. It also reduces the ability of red blood cells to carry oxygen throughout the body. The lack of oxygen can result in various symptoms such as fatigue, shortness of breath, and pain episodes called "crises".

It is important to note that sickle-cell anemia is not caused by abnormalities in white blood cells (option 1), thrombocytes (option 3), or blood plasma composition (option 4). White blood cells are responsible for fighting infections, thrombocytes are involved in blood clotting, and blood plasma is the liquid component of blood. However, sickle-cell anemia primarily affects the red blood cells.