Step 1: Understanding the Question:
An infant, under 1 year of age, who is choking on a foreign body needs a specific, ordered sequence of maneuvers, and doing them out of order can waste time or fail to clear the airway.
Step 2: Key Formula or Approach:
For an infant with a foreign body airway obstruction, the recommended sequence is: give back blows first, then chest thrusts, and only look inside the mouth to remove a now visible object using a tongue-jaw lift afterward. A blind finger sweep without first seeing the object is avoided, since it can push the object further down.
Step 3: Detailed Explanation:
Back blows come first: with the infant face down along the rescuer's forearm, up to five firm back blows are given between the shoulder blades to try to dislodge the object using force and gravity together.
If this fails, chest thrusts follow: the infant is turned face up, and up to five chest thrusts are given, similar in position to chest compressions, to create a sudden rise in chest pressure that can push the object out.
Only after these two steps does the rescuer look into the mouth, and a tongue-jaw lift is used to see and remove the object if it is now visible, rather than sweeping blindly.
This gives the order: back blows (3), then chest thrust (1), then tongue-jaw lift (2), written as 3, 1, 2.
Option (a), 1-3-2, wrongly starts with a chest thrust before back blows.
Option (b), 3-2-1, correctly starts with back blows but then checks the mouth before trying chest thrusts.
Option (d), 2-1-3, wrongly starts by looking in the mouth before any physical maneuver is even tried.
Step 4: Final Answer:
\[ \boxed{3,\ 1,\ 2\ (\text{Back blows} \to \text{Chest thrust} \to \text{Tongue-jaw lift})} \]