Fatty acids may be saturated or unsaturated.
Saturated fatty acids contain only single bonds between carbon atoms.
Unsaturated fatty acids contain one or more carbon-carbon double bonds:
\[
C=C.
\]
Palmitic acid is a saturated fatty acid.
Stearic acid is also a saturated fatty acid.
Oxalic acid is not a fatty acid.
Oleic acid contains one carbon-carbon double bond.
Therefore, oleic acid is a monounsaturated fatty acid.
Hence, the unsaturated fatty acid among the given options is:
\[
\text{Oleic acid}.
\]