Which of these blood samples most likely came from the left ventricle?

The answer would be Sample A (bright red blood cells).

Blood right ventricle is dark red because it has a high amount of carbon dioxide. The blood will be pumped into the lungs where carbon dioxide will diffuse out and oxygen will diffuse in. After that, the blood will become bright red and pumped into left atrium then left ventricle.

Blood in the left ventricle would be bright red because it contains a high amount of oxygen.

Sample A most likely came from the left ventricle, as this is the portion of the heart that receives freshly oxygenated blood to be pumped to different parts of the body. Oxygenated blood is blood that has had oxygen bounded to it by hemoglobin (a pigment made of iron found in red blood cells). The oxidation of this blood by the iron in the hemoglobin gives it the RBC bright red look. This blood containing the bright RBCs in transported to the left ventricle from the lung, after which it is spread throughout the body. Hence, in normal conditions of the heart, the Sample A most likely came from the left ventricle.