Chlorine casualties requiring immediate treatment are most likely suffering from:

Chlorine is a potent irritant and can lead to a range of respiratory conditions when inhaled. Among the options given, acute laryngospasm is particularly relevant in cases of chlorine exposure. This reaction occurs rapidly after inhalation of chlorine gas, as it can cause the vocal cords to spasm and close tightly, obstructing airflow and making it difficult for the individual to breathe. This physiological response is critical because it often requires immediate medical attention to secure the airway and restore normal breathing.

In contrast, while hypoxia, bronchospasm, and pulmonary edema are also potential consequences of chlorine exposure, they typically manifest in a slightly different manner or timeline. Hypoxia can occur as a result of inadequate oxygen supply due to respiratory obstruction or other complications. Bronchospasm may follow exposure but is not as immediate as acute laryngospasm. Pulmonary edema, although a serious condition that can result from chlorine inhalation, develops later as fluid accumulates in the lungs, and while requiring urgent treatment, is not the most immediate concern in terms of airway obstruction.

Thus, given the context of immediate treatment needs following chlorine exposure, acute laryngospasm stands out as the most direct and urgent condition requiring intervention.