Concept:
Blood doping is a method used to enhance athletic performance by increasing the oxygen-carrying capacity of blood. This is commonly achieved by increasing the number of red blood cells (RBCs), which transport oxygen to body tissues through hemoglobin.
Step 1: Understand the purpose of blood doping.
The main aim of blood doping is to improve oxygen delivery to muscles during physical activity.
Step 2: Identify the blood component responsible for oxygen transport.
Red blood cells contain hemoglobin, which carries oxygen throughout the body.
Therefore, red blood cells are injected into the bloodstream during blood doping.
Hence, the correct answer is:
\[
\boxed{(B)\ \text{Red Blood Cell}}
\]