Step 1: Recall what the carbylamine test actually checks.
The carbylamine reaction works only on a primary amine, $\text{R-NH}_2$ or $\text{Ar-NH}_2$. The amine is heated with chloroform and alcoholic potassium hydroxide.
Step 2: Track what happens to the nitrogen.
Both hydrogens on the $-\text{NH}_2$ group are removed, and a carbon atom triple bonded to nitrogen takes their place. This gives the isocyanide group, $-\text{NC}$. \[ \text{R-NH}_2 + \text{CHCl}_3 + 3\text{KOH} \rightarrow \text{R-NC} + 3\text{KCl} + 3\text{H}_2\text{O} \]
Step 3: Eliminate the other options.
A carboxylic acid or an aldehyde would need an oxygen atom to enter the product, and this reaction supplies none. Free cyanide ion is also not the isolated product; the carbon stays attached to nitrogen throughout.
Step 4: Connect the product to its known property.
The isocyanide formed has a sharp, foul smell, which is why chemists use this exact reaction as a quick test for primary amines.
\[ \boxed{\text{Major product} = \text{Isocyanide}} \]