Mesityl oxide is a common name of:

Mesityl oxide is a well-known organic compound formed by the aldol condensation of acetone. When two molecules of acetone undergo aldol condensation, they first form diacetone alcohol, which then undergoes dehydration to give mesityl oxide.

The product has the molecular formula $$C_6H_{10}O$$ and its structure is $$CH_3\text{-}CO\text{-}CH=C(CH_3)_2$$. To name it by IUPAC rules, we identify the longest carbon chain containing both the carbonyl group and the double bond. The longest such chain has 5 carbons (pent-), so the parent name is pentanone.

Numbering from the end nearer to the carbonyl group: carbon 1 is $$CH_3$$, carbon 2 carries the $$C=O$$ (keto group), carbon 3 is $$CH=$$, carbon 4 is $$=C(CH_3)$$, and carbon 5 is the terminal $$CH_3$$ attached to carbon 4. The double bond is between carbons 3 and 4, and there is a methyl branch at carbon 4.

Putting it together, the IUPAC name is 4-methylpent-3-en-2-one. This matches option (D).