Question

which compound can be oxidized to a carboxylic acid

Explanation:

Step1: Recall oxidation rule

Primary alcohols can be oxidized to carboxylic - acids.

Step2: Identify primary alcohols

A primary alcohol has the -OH group attached to a carbon that is bonded to only one other carbon atom.
The third structure (a straight - chain alcohol) is a primary alcohol. The first, second, fourth, and fifth structures are secondary alcohols (the -OH group is attached to a carbon bonded to two other carbon atoms).

Answer:

The third compound (the straight - chain alcohol with -OH at the end)