Answer:
The correct answer would be erythrocyte.
Specialized cell refers to a fully differentiated cell which performs a specific function.
Meristem refers to the undifferentiated cells present in plants which help in the growth of a plant.
Bacteria refers to a unicellular prokaryotic organism.
Erythrocyte or red blood cell is a fully differentiated cell which contains hemoglobin and helps in transportation of oxygen and carbon dioxide in organisms.