Step 1: Find product X.
When $C_2H_5Cl$ reacts with KCN, the cyanide attacks through its carbon end, giving the nitrile as the major product. So X is ethyl cyanide, $C_2H_5CN$ (propanenitrile).
Step 2: Test route I, the carbylamine reaction.
Heating $C_2H_5NH_2$ with $CHCl_3$ and KOH gives an isocyanide, $C_2H_5NC$, where nitrogen joins the carbon. That is the iso form, not the nitrile $C_2H_5CN$. So route I does NOT give X.
Step 3: Test route II, amide dehydration.
Heating $C_2H_5CONH_2$ with a dehydrating agent removes water and turns the amide into the nitrile $C_2H_5CN$. So route II gives X.
Step 4: Test route III, oxime dehydration.
$C_2H_5CHO$ first reacts with $NH_2OH$ to form an oxime, then acetic anhydride pulls out water to give the nitrile $C_2H_5CN$. So route III gives X.
Step 5: Collect the working routes.
Routes II and III both give the same nitrile, while route I gives the isocyanide instead.
Step 6: Conclusion.
So the answer is II, III only. \[ \boxed{\text{II, III only}} \]