To solve this problem, we need to understand how carbon dioxide is transported in the blood.$\newline$Approximately 70% of carbon dioxide absorbed by the blood is transported to the lungs as bicarbonate ions. Let's break down the process:$\newline \bullet$When carbon dioxide enters the blood, it reacts with water to form carbonic acid (H$_2$CO$_3$) in the presence of the enzyme carbonic anhydrase.$\newline \bullet$The carbonic acid then dissociates into bicarbonate ions (HCO$_3^-$) and hydrogen ions (H$^+$).$\newline \bullet$The bicarbonate ions are transported in the plasma, which accounts for the majority of carbon dioxide transport.$\newline$Now, let's evaluate the options:$\newline \bullet$Option 1: As bicarbonate ions - This is correct as it describes the primary method of carbon dioxide transport.$\newline \bullet$Option 2: In the form of dissolved gas molecules - Only a small percentage of carbon dioxide is transported this way.$\newline \bullet$Option 3: By binding to RBCs - Carbon dioxide can bind to hemoglobin, but this is not the primary method.$\newline \bullet$Option 4: As carbamino-haemoglobin - This accounts for a smaller portion of carbon dioxide transport.$\newline$Therefore, the correct answer is Option 1: As bicarbonate ions.