Answer:
The agent causing the pneumonia, where bluish-green pus was found, is most likely Pseudomonas aeruginosa.
Step-by-step explanation:
Pseudomona aeuriginosa is a gram-negative bacteria that is one of the main causes of hospital-acquired infections, including pneumonias in mechanically ventilated patients.
One of the characteristics of P. aeuriginosa is the formation of a bluish-green pus, since it has the capacity to form cyanide-based blue pigment upon contact with the organic tissues it infects. This is the reason why previously P. aeruginosa was called a pyocyanic bacillus.
The other options are not correct because the only bacterium that produces blue-green pus is P. aeruginosa.