AMBU stands for:

Correct option is D

AMBU (Artificial Manual Breathing Unit) is a self-inflating bag used for emergency manual ventilation. Explanation of options:
· (a) ✘ AMBU is not related to anesthesia machines.
· (b) ✘ "Mandatory" implies automation.
· (c) ✘ AMBU is manually operated.
· (d) ✔ Correct; full form.