Identify the principal functional group

In IUPAC nomenclature, the highest priority functional group gets the suffix.

Here, the compound contains:

• a carboxylic acid group $(-COOH)$
• an aldehyde group $(-CHO)$

Since carboxylic acid has higher priority than aldehyde, the parent name must be based on the carboxylic acid.

Choose the longest chain containing the carboxyl carbon

The parent chain has 6 carbons, so the base name is: $\text{hexanoic acid}$

Numbering starts from the carboxylic acid carbon as carbon 1.

Locate the substituents

From the structure implied by the options:

• there is a methyl substituent at carbon 2
• the terminal aldehyde carbon, when acid is principal group, is named by the prefix oxo if it lies within the main chain end counted from the acid side

Thus, at carbon 5 there is an oxo group.

Construct the name

Parent: $\text{hexanoic acid}$

Substituents:

• $2$-methyl
• $5$-oxo

Therefore the correct IUPAC name is: $\boxed{\text{2-Methyl-5-oxohexanoic acid}}$

Check options

• A: $2$-Formyl-$5$-methylhexan-$6$-oic acid
  Incorrect: carboxylic acid carbon must be carbon 1, not 6.

• B: $5$-Methyl-$2$-oxohexan-$6$-oic acid
  Incorrect numbering of carboxylic acid; acid carbon cannot be 6.

• C: $5$-Formyl-$2$-methylhexanoic acid
  Incorrect because terminal aldehyde in presence of higher priority acid is represented as oxo, not formyl here.

• D: $2$-Methyl-$5$-oxohexanoic acid
  Correct.

Comparison with stored answer

Stored correct answer: D
Derived answer: D

Hence, the derived answer agrees with the stored answer.