Step 1: Know what we are hunting for. We want the one reaction whose final product is butanoic acid, that is $\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$. This is a straight four carbon acid with the acid group sitting at the very end of the chain. So we just need to track the carbons in each option and see which one lands exactly here.
Step 2: Set the carbon counting rule. The cleanest trick is to count carbons before and after each reaction. Butanoic acid has four carbons in a row with no branching, and the COOH carbon must be a chain end. Any path that loses a carbon, branches the chain, or puts the acid group in the middle is automatically wrong.
Step 3: Reject the haloform path. A haloform reaction takes a methyl ketone and chops off the $\text{CH}_3$ carbon as $\text{CHX}_3$. That means the acid you get has one fewer carbon than the starting ketone, and it is shortened from the carbonyl end. This usually gives the wrong carbon count, so a methyl ketone of this size will not cleanly give the four carbon straight acid we need.
Step 4: Reject oxidation that mismatches the chain. Oxidising a primary alcohol or an aldehyde keeps the carbon count the same and pins the acid group at the end. So the correct option is the one where a four carbon primary alcohol or a four carbon aldehyde (butan-1-ol or butanal) is oxidised, because that gives exactly $\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$ with no carbon lost.
Step 5: Reject hydrolysis or branching paths. Any route that hydrolyses a nitrile of the wrong length, or that comes from a branched skeleton, will give either a shorter acid, a longer acid, or a branched acid like 2-methylpropanoic acid, not the clean straight chain. Those are eliminated.
Step 6: Confirm the winner. Only the pathway that simply oxidises a straight four carbon primary alcohol or aldehyde, or that builds a clean four carbon chain ending in COOH, gives true butanoic acid. That is option A.
\[ \boxed{\text{Option A produces butanoic acid, } \text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}} \]
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