Recall that volatility is inversely proportional to the boiling point. The boiling point itself is directly proportional to the intermolecular forces. In the above case, we have two major intermolecular forces: H-bonds and dipole-dipole attractions.
Now, it is factual info that ammonia's intermolecular forces surpass even stibine's intermolecular attractions, so its volatility is the least.
Thus, we are only left with phosphine, arsine, and stibine. Here, recall that intermolecular forces are van Der Waal's forces, and thus their magnitude is directly to their size. (read the "why" of it here). Hence, stibine is the second-least volatile (as it has maximum van der waal's force due to its bigger size), followed by asrine, and finally phosphine.