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A one third normal saline solution is defined as a solution with a sodium chloride (NaCl) concentration that is one third the concentration of normal saline. Normal saline is a solution that contains 0.9% NaCl, which is isotonic with human blood plasma. Therefore, to determine the concentration of a one third normal saline solution, you take one third of 0.9%.

Calculating this gives:

0.9% NaCl ÷ 3 = 0.3% NaCl.

However, in the context of available options and common medical terminology, the designation "one third normal saline" refers to a 0.33% NaCl solution, which is the same proportion expressed in simpler terms. It is important in clinical practice to understand these concentrations for proper intravenous fluid administration, as different saline concentrations can have diverse physiological effects.

In summary, one third normal saline is recognized as a 0.33% NaCl solution, making it the correct answer.