To determine which respiratory volume of the human lung is the lowest, we must understand the different respiratory volumes:

1. Tidal Volume (TV): This is the amount of air inhaled or exhaled during a normal breath at rest. It is typically around 500 ml in an adult.
2. Residual Volume (RV): This is the amount of air remaining in the lungs after a forceful exhalation. It normally measures about 1200 ml.
3. Inspiratory Reserve Volume (IRV): This is the additional amount of air that can be inhaled with maximum effort after a normal inhalation. It typically ranges from 1900 to 3100 ml, depending on age, gender, and physical condition.
4. Expiratory Reserve Volume (ERV): This is the additional amount of air that can be exhaled with maximum effort after a normal exhalation. It is usually about 1000 ml.

Given these definitions and average values, it becomes clear that:

• The Tidal Volume (500 ml) is the smallest when compared to the other respiratory volumes.
• Residual Volume, Inspiratory Reserve Volume, and Expiratory Reserve Volume are all higher than the Tidal Volume in their average measurements.

The correct answer is Tidal Volume, as it has the lowest value among the listed respiratory volumes of the human lung.