Final answer:
The Correct option is 3). The air indicated by the student's plot likely results from using an incorrect CO₂ concentration, incorrect volume of CO₂ solution, or improper mixing of the solution, rather than just a plotting error.
Step-by-step explanation:
The most likely cause of the air in the point plotted by the student on the left side of the standard curve for 0.50 molar CO₂⁺ could be due to a number of factors. It could be because the student used an incorrect concentration of CO₂, made an error in plotting the point, used an incorrect volume of CO₂ solution, or did not properly mix the CO₂ solution. If the point is significantly off from where it should be according to the standard curve, then it's less likely to be a simple plotting error and more likely due to an error in the preparation or measurement of the CO₂ solution.