Breathing and Exchange Questions

A large proportion of oxygen is left unused in the human blood even after its uptake by the body tissues. This $O_2$:

Two friends are eating together on a dining table. One of them suddenly starts coughing while swallowing some food. This coughing would have been due to improper movement of:

The figure given below shows a small part of human lung where exchange of gases takes place. Which part is correctly identified?