Concept: Air path in order.
Air moves through the respiratory tract in a fixed order: trachea, then bronchi, then bronchioles, then terminal bronchioles, then respiratory bronchioles, then alveolar ducts, then alveolar sacs, then alveoli.
Step 1: Place respiratory bronchioles in this chain.
Respiratory bronchioles sit right after the terminal bronchioles. The word "respiratory" tells us these are the first tubes where a little gas exchange starts, because a few alveoli poke out of their walls.
Step 2: Find what comes right after them.
The next structure in the chain, right after the respiratory bronchioles, is the alveolar duct. Alveolar ducts are almost fully lined with alveoli, so gas exchange is much higher here than in the respiratory bronchiole itself.
Step 3: Rule out the other three options.
Tertiary bronchi and primary bronchi both come much earlier in the tree, before any bronchioles, and belong purely to the conducting zone with no gas exchange. Alveolar sacs come one step later, after the alveolar ducts, not right after the respiratory bronchioles.
Final Answer:
Respiratory bronchioles terminate into alveolar ducts, option (3).